UV-UV Hole-burning Spectroscopy Research Articles

N···C═O n → π* Interaction: Gas-Phase Electronic and Vibrational Spectroscopy Combined with Quantum Chemistry Calculations.

Herein, we have used gas-phase electronic and vibrational spectroscopic techniques for the first time to study the N···C═O n → π* interaction in ethyl 2-(2-(dimethylamino) phenyl) acetate (NMe2-Ph-EA). We have measured the electronic spectra of NMe2-Ph-EA in the mass channels of its two distinct fragments of m/z = 15 and 192 using a resonant two-photon ionization technique as there was extensive photofragmentation of NMe2-Ph-EA. Identical electronic spectra obtained in the mass channels of both fragments confirm the dissociation of NMe2-Ph-EA in the ionic state, and hence, the electronic spectrum of the fragment represents that of NMe2-Ph-EA only. UV-UV hole-burning spectroscopy proved the presence of a single conformer of NMe2-Ph-EA in the experiment. Detailed quantum chemistry calculations reveal the existence of a N···C═O n → π* interaction in all six low-energy conformers of NMe2-Ph-EA. A comparison of the IR spectrum of NMe2-Ph-EA acquired from the gas-phase experiment with those obtained from theoretical calculations indicates that the experimentally observed conformer has a N···C═O n → π* interaction. The present finding might be further valuable in drug design and their recognition based on the N···C═O n → π* interaction.

New aspect of photophysics of 7,7,8,8-tetracyanoquinodimethane and its solvated complexes: intra- vs. inter-molecular charge-transfer.

We performed laser-induced fluorescence (LIF) spectroscopy of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its solvated complexes with acetonitrile (ACN) and benzene (Bz), under the jet-cooled gas-phase condition. We also carried out fluorescence and its time profile measurements in TCNQ/Bz/hexane solution to compare with the gas-phase results. The LIF excitation spectrum of the S1 (ππ*)–S0 electronic transition of TCNQ monomer exhibited unusual vibronic structure with the maximum intensity at ∼3000 cm−1 above the band origin. In addition, the fluorescence lifetime is more than 100 times longer than that in hexane solution with most of the bands showing double exponential decay. The unusual feature of the vibronic bands is intrinsic and not due to the presence of other species, as confirmed by UV-UV hole-burning (HB) spectroscopy. These unusual features of S1 are thought to be due to the coupling with the S2 state, where S2 was revealed to have intramolecular charge-transfer (ICT) character. The S1–S0 transition of the TCNQ–ACN complex exhibited sharp vibronic bands which are red-shifted by 120 cm−1 from those of the monomer, indicating van der Waals (vdW) interaction between them; however, the fluorescence lifetime was drastically shortened. In contrast, the TCNQ–Bz complex gave a broad electronic spectrum. The study of the fluorescence and its time profile in TCNQ/Bz/hexane solution clearly shows the formation of the CT complex between TCNQ and Bz. Based on the experimental results and density functional theory (DFT) calculations, we propose that in the TCNQ monomer and TCNQ–ACN complex the S1 (ππ*) state is coupled to the intramolecular CT state, while the S1 state of TCNQ in the TCNQ–Bz complex is more strongly coupled to the intermolecular CT state.

Electronic State and Photophysics of 2-Ethylhexyl-4-methoxycinnamate as UV-B Sunscreen under Jet-Cooled Condition.

The title compound, 2-ethylhexyl-4-methoxycinnamate (2EH4MC), is known as a typical ingredient of sunscreen cosmetics that effectively converts the absorbed UV-B light to thermal energy. This energy conversion process includes the nonradiative decay (NRD): trans-cis isomerization and finally going back to the original structure with a release of thermal energy. In this study, we performed UV spectroscopy for jet-cooled 2EH4MC to investigate the electronic/geometrical structures as well as the NRD mechanism. Laser-induced-fluorescence (LIF) spectroscopy gave the well-resolved vibronic structure of the S1-S0 transition; UV-UV hole-burning (HB) spectroscopy and density functional theory (DFT) calculations revealed the presence of syn and anti isomers, where the methoxy (-OCH3) groups orient in opposite directions to each other. Picosecond UV-UV pump-probe spectroscopy revealed the NRD process from the excited singlet (S1 (1ππ*)) state occurs at a rate constant of ∼1010-1011 s-1, attributed to internal conversion (IC) to the 1nπ* state. Nanosecond UV-deep UV (DUV) pump-probe spectroscopy identified a transient triplet (T1 (3ππ*)) state, whose energy (from S0) and lifetime are 18 400 cm-1 and 20 ns, respectively. These results demonstrate that the photoisomerization of 2EH4MC includes multistep internal conversions and intersystem crossings, described as "S1 (trans, 1ππ*) → 1nπ* → T1 (3ππ*) → S0 (cis)".

Rotamers of p‑isopropylphenol studied by hole-burning resonantly enhanced multiphoton ionization and mass analyzed threshold ionization spectroscopy.

The resonance enhanced multiphoton ionization (REMPI), ultraviolet-ultraviolet (UV-UV) hole burning and mass analyzed threshold ionization (MATI) spectroscopy have been applied to investigate the vibrational features of p‑isopropylphenol in its first electronically excited state S1 and cationic ground state D0. Two stable conformational structures of p‑isopropylphenol are distinctly found in the supersonic molecular beam and identified as the cis and trans rotamers through REMPI and UV-UV hole burning spectroscopy. The electronic excitation energies of S1 ← S0 transition of two rotamers are determined to be 35,578 and 35,593 cm-1, and the adiabatic ionization energies are 65,331 and 65,350 cm-1, respectively. The MATI spectra recorded via different intermediate levels of S1 state indicate the similarity in the molecular geometry between the S1 state and the D0 state for each rotamer of p‑isopropylphenol. Geometrical optimizations of p‑isopropylphenol have also been performed using the density functional theory (DFT) for S0 and D0 states, and time-dependent density functional theory (TDDFT) for S1 state. The simulated spectra for S1 ← S0 and D0 ← S1 transitions of two rotamers are able to reproduce qualitatively the experimental spectral profile, which help us to assign the vibronic modes. Most of the observed vibrations of two rotamers in the S1 and D0 states are related to the in-plane ring deformation and some active modes involving isopropyl group.

Entropic effects make a more tightly folded conformer of a β-amino acid less stable: UV-UV hole burning and IR dip spectroscopy of l-β3-homotryptophan using a laser desorption supersonic jet technique.

UV-UV hole burning and IR dip spectra of l-β3-homotryptophan were measured by a laser desorption supersonic jet technique as a bottom-up approach to understand the secondary structures of β-peptides. 14 conformers were found by UV-UV hole burning spectroscopy. The conformers were classified into three groups depending on their hydrogen bonding patterns observed in their conformer-specific IR spectra, and tentatively assigned by comparing with quantum chemical calculations. Group 1 had free OH stretch but no NH2 anti-symmetric stretch vibrational transition and was assigned to NH-π hydrogen bonded structures. Group 2, including the most abundant conformer, showed both free OH and NH2 anti-symmetric stretch vibrations, and belonged to NH-O hydrogen bonded conformations. Group 3 of conformers had hydrogen-bonded OH stretch IR transition and had OH-N hydrogen bonds. The internal hydrogen bond of group 3 is a C6 hydrogen bond due to the additional carbon atom at the β position and shows a shorter bond length than that of a C5 hydrogen bond. While the OH-N C6 hydrogen bond is stronger than NH-O, the entropic effect prefers the more flexible NH-O hydrogen bonded structure. It is expected that the unnatural C6 hydrogen bond influences the conformations of β-peptides and builds totally different secondary structures than those of α-peptides.

High-cooling-efficiency cryogenic quadrupole ion trap and UV-UV hole burning spectroscopy of protonated tyrosine

The cooling efficiency of a cryogenic three-dimensional quadrupole ion trap (QIT) is drastically improved by using copper electrodes instead of conventional stainless-steel ones. The temperature of trapped ions (protonated tyrosine TyrH+) was estimated based on the ultraviolet (UV) photo-dissociation spectra. The UV spectrum of TryH+ shows almost no hot bands, and thus the high cooling efficiency of the copper ion trap was proven. The temperature was also estimated by simulating the observed band contour in the UV spectra, which is determined by the population in the rotationally excited levels. From the simulations, the temperature of TryH+ was estimated to be ∼13K, while that in the stainless-steel QIT was 45–50K. In addition, to demonstrate the advantage of the copper QIT, UV-UV hole burning (HB) spectra, i.e. conformation-selected UV spectra, were measured. It was confirmed that four different conformers, A∼D, coexist in the ultra-cold protonated tyrosine. By comparing with the calculated Franck-Condon spectra, their structural assignments were discussed, including the orientation of the OH group.

Conformation of Alkali Metal Ion-Benzo-12-Crown-4 Complexes Investigated by UV Photodissociation and UV-UV Hole-Burning Spectroscopy.

We measure UV photodissociation (UVPD) spectra of benzo-12-crown-4 (B12C4) complexes with alkali metal ions, M(+)·B12C4 (M = Li, Na, K, Rb, and Cs), in the 36300-37600 cm(-1) region. Thanks to the cooling of ions to ∼10 K, all the M(+)·B12C4 complexes show sharp vibronic bands in this region. For UV-UV hole-burning (HB) spectroscopy, we first check if our experimental system works well by observing UV-UV HB spectra of the K(+) complex with benzo-18-crown-6 (B18C6), K(+)·B18C6. In the UV-UV HB spectra of the K(+)·B18C6 complex, gain signals are also observed; these are due to vibrationally hot K(+)·B18C6 complex produced by the UV excitation of cold K(+)·B18C6 complex. Then we apply UV-UV HB spectroscopy to the M(+)·B12C4 complexes, and only one conformer is found for each complex except for the Li(+) complex, which has two conformers. The vibronic structure around the origin band of the UVPD spectra is quite similar for all the complexes, indicating close resemblance of the complex structure. The most stable structures calculated for the M(+)·B12C4 (M = Li, Na, K, Rb, and Cs) complexes also have a similar conformation among them, which coincides with the UVPD results. In these conformers the metal ions are too big to be included in the B12C4 cavity, even for the Li(+) ion. In solution, it was reported that 12-crown-4 (12C4) shows the preference of Na(+) ion among alkali metal ions. From the similarity of the structure for the M(+)·B12C4 complexes, it is suggested that the solvation of free metal ions, not of the M(+)·12C4 complexes, may lead to the selectivity of Na(+) ion for 12C4 in solution.

Experimental and theoretical study on the excited-state dynamics of ortho-, meta-, and para-methoxy methylcinnamate.

The S1 state dynamics of methoxy methylcinnamate (MMC) has been investigated under supersonic jet-cooled conditions. The vibrationally resolved S1-S0 absorption spectrum was recorded by laser induced fluorescence and mass-resolved resonant two-photon ionization spectroscopy and separated into conformers by UV-UV hole-burning (UV-UV HB) spectroscopy. The S1 lifetime measurements revealed different dynamics of para-methoxy methylcinnamate from ortho-methoxy methylcinnamate and meta-methoxy methylcinnamate (hereafter, abbreviated as p-, o-, and m-MMCs, respectively). The lifetimes of o-MMC and m-MMC are on the nanosecond time scale and exhibit little tendency of excess energy dependence. On the other hand, p-MMC decays much faster and its lifetime is conformer and excess energy dependent. In addition, the p-MMC-H2O complex was studied to explore the effect of hydration on the S1 state dynamics of p-MMC, and it was found that the hydration significantly accelerates the nonradiative decay. Quantum chemical calculation was employed to search the major decay route from S1(ππ(∗)) for three MMCs and p-MMC-H2O in terms of (i) trans → cis isomerization and (ii) internal conversion to the (1)nπ(∗) state. In o-MMC and m-MMC, the large energy barrier is created for the nonradiative decay along (i) the double-bond twisting coordinate (∼1000 cm(-1)) in S1 as well as (ii) the linear interpolating internal coordinate (∼1000 cm(-1)) from S1 to (1)nπ(∗) states. The calculation on p-MMC decay dynamics suggests that both (i) and (ii) are available due to small energy barrier, i.e., 160 cm(-1) by the double-bond twisting and 390 cm(-1) by the potential energy crossing. The hydration of p-MMC raises the energy barrier of the IC route to the S1/(1)nπ(∗) conical intersection, convincing that the direct isomerization is more likely to occur.

New Method for Double-Resonance Spectroscopy in a Cold Quadrupole Ion Trap and Its Application to UV–UV Hole-Burning Spectroscopy of Protonated Adenine Dimer

A novel method for double-resonance spectroscopy in a cold quadrupole ion trap is presented, which utilizes dipolar resonant excitation of fragment ions in the quadrupole ion trap. Photofragments by a burn laser are removed by applying an auxiliary RF to the trap, and a probe laser detects the depletion of photofragments by the burn laser. By scanning the wavelength of the burn laser, conformation-specific UV spectrum of a cold ion is obtained. This simple and powerful method is applicable to any type of double-resonance spectroscopy in a cold quadrupole ion trap and was applied to UV-UV hole-burning spectroscopy of protonated adenine dimer. It was found that protonated adenine dimer has multiple conformers/tautomers, each with multiple excited states with drastically different excited state dynamics.

Single-conformation UV and IR spectroscopy of model G-type lignin dilignols: the β–O–4 and β–β linkages

Single-conformation ultraviolet and infrared spectroscopy was performed on dilignols containing two of the three biologically prevalent β-lignol linkages, erythro β–O–4 (β-aryl ether) and (±) β–β (pinoresinol). Both dilignols contain guaiacol(G)-type sub-units, representative of these linkages in G-type lignin. Resonant two-photon ionization (R2PI), IR-UV, and UV-UV holeburning (UVHB) spectroscopy in the cold, isolated environment of a supersonic expansion was carried out to determine the spectroscopic signatures associated with each linkage conformation, revealing striking differences in the vibronic intensity patterns between the two molecules in the UV. Two conformational isomers were found for the β–O–4 dilignol, both being classified into the fully hydrogen-bonded family with α-OH⋯OCH3 (C8) and γ-OH⋯Oβ (C5) H-bonds that are characteristic of the β–O–4 linkage. Conversely, a single dominant conformation was found for the conformationally-constrained pinoresinol. Resonant ion-dip infrared (RIDIR) spectroscopy provided conformation-specific IR spectra in the OH stretch and alkyl CH stretch regions, yielding complementary data that reported on both the intramolecular H-bonding and more subtle linkage features, respectively. DFT M05-2X calculations predict that the rigid β–β linkage had far fewer low-energy conformations in the first 20 kJ mol−1 (3) than the more flexible β–O–4 linkage (45). In the β–O–4 lignin dimer, the distinct UV chromophores lead to a splitting between S1 and S2 states that is determined mainly by the differences in the chemical structures of the two chromophores. In pinoresinol however, the assigned structure has C2 symmetry, with a calculated vertical excitonic splitting between the S1 and S2 states of 74 cm−1 (TDDFT). After taking into account the reduction in splitting associated with the geometry change in the aromatic rings upon electronic excitation, a vibronically quenched excitonic splitting of no more than a few wavenumbers is predicted for the C2 symmetric pinoresinol, but a definite experimental confirmation was not possible. These results predict that, under most circumstances, adjacent chromophores along a lignin polymer chain are not significantly electronically coupled to one another, and can be treated largely as isolated chromophores.

Conformationally resolved spectra of acetaminophen by UV-UV hole burning and IR dip spectroscopy in the gas phase

Electronic and vibrational spectra of acetaminophen were measured by using UV-UV hole burning (HB) and IR dip spectroscopy. HB spectra show the coexistence of 4 different species, which include two new ones. Low-frequency transitions in the spectra are reproduced by a one-dimensional periodic potential with a free-rotor basis set for the methyl group. From the analysis, we concluded that acetaminophen has two conformers and each conformer gives two independent transitions starting from the most stable 0a(1) and the hot 1e internal rotational levels. It is also found that the HB spectrum of the trans-conformer in the previous report is that from the 1e excited level, while the HB spectrum of the cis-conformer is contaminated by the transitions of the trans-conformer. Potential curves of the methyl rotational motion are determined both in S(0) and S(1).

Conformationally resolved structures of jet-cooled acetaminophen by UV–UV hole-burning spectroscopy

The conformational structures of jet-cooled acetaminophen were investigated in the gas phase by resonant 2-photon ionization and UV-UV hole-burning spectroscopy. In contrast to the results from a previous study, two nearly isoenergetic conformers were distinctly found in a supersonic molecular beam expansion and positively identified as the cis and trans isomers of acetaminophen by UV-UV hole-burning spectroscopy. The 0-0 bands of the cis and trans isomers were found at 33518.7 and 33485.6 cm(-1), respectively. The vibronic bands of the two isomers are close-lying and/or partially overlapping due to the small energy difference (33 cm(-1)) between the two 0-0 bands. As a consequence, the recorded resonant 2-photon ionization spectrum is highly congested in the low excitation energy region, which develops continuously into a featureless, broadened spectrum in the high energy region.

Structure of host–guest complexes between dibenzo-18-crown-6 and water, ammonia, methanol, and acetylene: Evidence of molecular recognition on the complexation

Complexes of dibenzo-18-crown-6 (DB18C6, host) with water, ammonia, methanol, and acetylene (guest) in supersonic jets have been characterized by laser induced fluorescence (LIF), UV-UV hole-burning (UV-UV HB), and IR-UV double resonance (IR-UV DR) spectroscopy. Firstly, we reinvestigated the conformation of bare DB18C6 (species m1 and m2) and the structure of DB18C6-H(2)O (species a) [R. Kusaka, Y. Inokuchi, T. Ebata, Phys. Chem. Chem. Phys., 2008, 10, 6238] by measuring IR-UV DR spectra in the region of the methylene CH stretching vibrations. The IR spectral feature of the methylene CH stretch of DB18C6-H(2)O is clearly different from those of bare DB18C6 conformers, suggesting that DB18C6 changes its conformation when forming a complex with a water molecule. With the aid of Monte Carlo simulation for extensive conformational search and density functional calculations (M05-2X/6-31+G*), we reassigned species m1 and m2 to conformers having C(1) and C(2) symmetry, respectively. Also, we confirmed the DB18C6 part in species a of DB18C6-H(2)O to be "boat" conformation (C(2v)). Secondly, we identified nine, one, and two species for the DB18C6 complexes with ammonia, methanol, and acetylene, respectively, by the combination of LIF and UV-UV HB spectroscopy. From the IR spectroscopic measurement in the methylene CH stretching region, a similar conformational change was identified in the DB18C6-ammonia complexes, but not in the complexes with methanol or acetylene. The structures of all the complexes were determined by analyzing the electronic transition energies, exciton splitting, and IR spectra in the region of the OH, NH, and CH stretching vibrations. In DB18C6-ammonia complexes, an ammonia molecule is incorporated into the cavity of the boat conformation by forming "bifurcated" and "bidentate" hydrogen-bond (H-bond), similar to the case of the DB18C6-H(2)O complex. On the other hand, in the DB18C6-methanol and -acetylene complexes, methanol and acetylene molecules are simply attached to the C(1) and C(2) conformations, respectively. From the difference of the DB18C6 conformations depending on the type of the guest molecules, it is concluded that DB18C6 distinguishes water and ammonia from methanol and acetylene when it forms complexes, depending on whether guest molecules have an ability to form bidentate H-bonding.

Remarkable Site Difference of Vibrational Energy Relaxation in Benzene Dimer: Picosecond Time‐Resolved IR–UV Pump–Probe Spectroscopy

Benzene dimer (Bz2) is one of the most fundamental molecular systems and shows intriguing properties attributed to aromaticaromatic weak interactions. The structure of Bz2 in the ground electronic state (S0) has been extensively studied experimentally and theoretically. Several experimental studies on Bz2 such as the molecular-beam electric deflection 1(b)] and Raman-UV double resonance and UV-UV hole-burning spectroscopy strongly suggested that the two benzene (Bz) molecules are not symmetrically equivalent. In addition, recent high level ab initio calculations suggested that a floppy T-shaped structure is a global minimum, although there are several possible isomers in nearly isoenergetic region. The rotational constant obtained from the Fourier-transform microwave spectrum agreed well with the predicted value for the T-shaped form. An important subject strongly related to the structure is the vibrational dynamics. In the T-shaped Bz2, the two Bz molecules are at different symmetrical conditions; one is at the Stem-site, and the other is at the Top-site. Therefore, the rate constant as well as the process of the vibrational energy relaxation of the two molecules will be different. In this sense, the study of a site-specified vibrational energy relaxation will provide us with a more detailed picture of the structure and vibrational dynamics of Bz2. Several studies on the vibrational energy relaxation of Bz2 have been reported. Lee and co-workers estimated the lifetime of the CH stretching region to be ~3 ps based on the band-width measurement of the IR spectrum. Felker and co-workers suggested that the 2 (CH stretching, Wilson’s numbering) level of the Stem-site Bz relaxes faster than that of the Top-site Bz, according to the difference in the band-width of the Raman spectra. However, in both cases the measured band-width was not that of a single rovibrational level. It is also not clear how the band-width is related to the vibrational energy relaxation dynamics, such as intramolecular (intracluster) vibrational energy redistribution (IVR) and vibrational predissociation (VP). Moreover, there has been no report on the time-resolved spectroscopy for the site-specified Bz2. Here we study the dynamics of the vibrational energy relaxation of the T-shaped Bz2 in the CH stretching energy region by using picosecond time-resolved IR-UV pump-probe spectroscopy. It is known that for the isolated Bz monomer having a D6h symmetry the IR active CH vibration (20) with the e1u symmetry is anharmonically coupled with the 8+19 and 1+6+19 Figure 1. Energy level diagram of isotope-substituted benzene dimers [h(Stem)d(Top) and d(Stem)h(Top)] and an excitation scheme. The dimers are vibrationally excited to the CH stretching region by a picosecond IR pulse, and the time evolution is probed by a picosecond UV pulse. The h*(Stem) and h*(Top) benzene components (an asterisk represents the vibrationally excited site) show different vibronic structures. Thus, the vibrationally excited dimers can be separately monitored by a tunable picosecond UV pulse. In this figure, VER means vibrational energy relaxation.

Laser spectroscopic study on (dibenzo-24-crown-8-ether)–water and –methanol complexes in supersonic jets

The conformation and complex formation with guest molecules have been investigated for jet-cooled dibenzo-24-crown-8-ether (DB24C8) by laser induced fluorescence (LIF), UV-UV hole-burning (UV-UV HB), and IR-UV double-resonance (IR-UV DR) spectroscopy. The results are compared with the results of dibenzo-18-crown-6-ether (DB18C6) and analyzed by density functional theory (DFT) calculations at the B3LYP/6-31+G* level. Five conformers are identified for DB24C8 under jet-cooled conditions, and the structure of the major isomer is determined to be a boat form, similar to the case of DB18C6. Two molecules (water and methanol) are investigated as the guest species for the encapsulation experiment. DB24C8 hardly encapsulates a water molecule, different from the case of DB18C6 in which a water molecule is efficiently captured. However, it is likely that larger (water)(n) clusters can be captured in the DB24C8 cavity. The different feature in the complex formation between DB24C8 and DB24C8 with the water molecules are attributed to a larger cavity size of DB24C8 than that of DB18C6. For methanol, two kinds of DB18C6-(methanol)(1) isomers are identified by IR-UV DR spectroscopy; one is the "OH...pi" H-bonded isomer, characteristic of methanol with one OH group, and the other is the "OH...O" H-bonded ones. These results indicate the multiple H-bond formation to the oxygen atoms of the ether ring may play an important role in the complexes of DB24C8 or DB18C6 with water.

Jet-cooled vibronic spectroscopy and asymmetric torsional potentials of phenylcyclopentene

The ultraviolet spectroscopy of the S(1) <-- S(0) transition of 1-phenylcyclopentene (PCP) was studied by resonant-two-photon ionization (R2PI), laser-induced fluorescence (LIF) and single vibronic level fluorescence (SVLF). UV-UV hole-burning (UVHB) spectroscopy was used to determine that there is only one spectroscopically distinct conformer in the supersonic expansion. The excitation spectrum shows extensive vibronic structure extending to over 1000 cm(-1) above the electronic origin (34,646 cm(-1)). Much of the vibronic structure is similar to that of styrene and other singly substituted benzene derivatives, with Franck-Condon (FC) activity predominantly in substituent-sensitive benzene modes. Sizeable FC progressions were also found in the inter-ring torsion, reflecting a large displacement in the inter-ring angle upon electronic excitation. No evidence for FC activity in the ring-puckering coordinate is observed. The torsional potentials of the ground and excited states were determined from the experimental transition frequencies by fitting the calculated to the experimental torsional frequency spacings in an automated least-squares fitting procedure. The S(1) torsional potential is a symmetric single-well potential centered around a locally planar equilibrium geometry at a torsional angle of varphi = 0 degrees . The energy levels are reproduced by a cosine term potential function with torsional parameters V(2) = 3765 cm(-1) and V(4) = -183 cm(-1). The S(0) torsional potential possesses a twisted equilibrium geometry that is strongly asymmetric about varphi = 0 degrees due to the non-planarity of the cyclopentene ring. The best-fit potential parameters uses a sin/cos potential function (odd/even), with V = 948 cm(-1), V = -195 cm(-1), V = -162 cm(-1) and V = -268 cm(-1). The shape of the potentials are similar to those predicted by relaxed potential energy scans calculated at the DFT, CIS and TDDFT//CIS levels of theory. The change in the torsional angle varphi upon electronic excitation was determined to be approximately 15 degrees from fits of the displacement delta of the S(0) torsional potential with respect to the S(1) potential. The simulated shift of the S(0) potential with respect to the S(1) potential of approximately 15 degrees is in very good agreement with that obtained from B3LYP calculations.

Single-Conformation Ultraviolet and Infrared Spectroscopy of Model Synthetic Foldamers: β-Peptides Ac-β3-hPhe-NHMe and Ac-β3-hTyr-NHMe

The conformational preferences and infrared and ultraviolet spectral signatures of two model beta-peptides, Ac-beta3-hPhe-NHMe (1) and Ac-beta3-hTyr-NHMe (2), have been explored under jet-cooled, isolated molecule conditions. The mass-resolved, resonant two-photon ionization spectra of the two molecules were recorded in the region of the S0-S1 origin of the phenyl or phenol ring substituents, respectively. UV-UV hole-burning spectroscopy was used to determine that two conformations of 1 are present, with the transitions due to conformer A, with S0-S1 origin at 34431 cm(-1), being almost 20 times larger than those due to conformer B, with S0-S1 origin at 34404 cm(-1). Only one conformation of 2 was observed. Resonant ion-dip infrared spectroscopy provided single-conformation infrared spectra in the 3300-3700 cm(-1) region. The spectra of conformer A of both molecules have H-bonded and free amide NH stretch infrared transitions at 3400 and 3488 cm(-1), respectively, while conformer B of 1 possesses bands at 3417 and 3454 cm(-1). For comparison with experiment, full optimizations of all low-lying minima of 1 were carried out at the DFT B3LYP/6-31+G* and RIMP2/aug-cc-pVDZ levels of theory, and single point MP2/6-31+G* calculations at the DFT geometries. On the basis of the comparison with previous studies in solution and the calculated results, conformer A of 1 and 2 were assigned to a C6 conformer, while conformer B of 1 was assigned to a unique C8 structure with a weak intramolecular H-bond. The reasons for the preference for C6 over C8 structures and the presence of only two conformations in the jet-cooled spectrum are discussed in light of the predictions from calculations.

Single-Conformation Ultraviolet and Infrared Spectroscopy of Model Synthetic Foldamers: β-Peptides Ac-β3-hPhe-β3-hAla-NHMe and Ac-β3-hAla-β3-hPhe-NHMe

The conformational preferences and infrared and ultraviolet spectral signatures of two model beta-peptides, Ac-beta3-hPhe-beta3-hAla-NHMe (1) and Ac-beta3-hAla-beta3-hPhe-NHMe (2), have been explored under jet-cooled, isolated-molecule conditions. The mass-resolved, resonant two-photon ionization spectra of the two molecules were recorded in the region of the S0-S1 origin of the phenyl substituents (37,200-37,800 cm(-1)). UV-UV hole-burning spectroscopy was used to determine the ultraviolet spectral signatures of five conformational isomers of both 1 and 2. Transitions due to two conformers (labeled A and B) dominate the R2PI spectra of each molecule, while the other three are minor conformers (C-E) with transitions a factor of 3-5 smaller. Resonant ion-dip infrared spectroscopy was used to obtain single-conformation infrared spectra in the 3300-3700 cm(-1) region. The infrared spectra showed patterns of NH stretch transitions characteristic of the number and type of intramolecular H-bonds present in the beta-peptide backbone. For comparison with experiment, full optimizations of low-lying minima of both molecules were carried out at DFT B3LYP/6-31+G*, followed by single point MP2/6-31+G* and selected MP2/aug-cc-pVDZ calculations at the DFT optimized geometries. Calculated harmonic vibrational frequencies and infrared intensities for the amide NH stretch vibrations were used to determine the beta-peptide backbone structures for nine of the ten observed conformers. Conformers 1B, 1D, and 2A were assigned to double ring structures containing two C6 H-bonded rings (C6a/C6a), conformers 1A and 2B are C10 single H-bonded rings, conformers 1C and 2D are double ring structures composed of two C8 H-bonded rings (C8/C8), and conformers 1E and 2E are double ring/double acceptor structures in which two NH groups H-bond to the same C=O group, thereby weakening both H-bonds. Both 1E and 2E are tentatively assigned to C6/C8 double ring/double acceptor structures, although C8/C12 structures cannot be ruled out unequivocally. Finally, no firm conformational assignment has been made for conformer 2C whose unusual infrared spectrum contains one very strong H-bond with NH stretch frequency at 3309 cm(-1), a second H-bonded NH stretch fundamental of more typical value (3399 cm(-1)), and a third fundamental at 3440 cm(-1), below that typical of a branched-chain free NH. The single conformation spectra provide characteristic wavenumber ranges for the amide NH stretch fundamentals ascribed to C6 (3378-3415 cm(-1)), C8 (3339-3369 cm(-1)), and C10 (3381-3390 cm(-1)) H-bonded rings.

Structure of hydrated clusters of dibenzo-18-crown-6-ether in a supersonic jet—encapsulation of water molecules in the crown cavity

The structure of dibenzo-18-crown-6-ether (DB18C6) and its hydrated clusters has been investigated in a supersonic jet. Two conformers of bare DB18C6 and six hydrated clusters (DB18C6-(H(2)O)(n)) were identified by laser-induced fluorescence, fluorescence-detected UV-UV hole-burning and IR-UV double-resonance spectroscopy. The IR-UV double resonance spectra were compared with the IR spectra obtained by quantum chemical calculations at the B3LYP/6-31+G* level. The two conformers of bare DB18C6 are assigned to "boat" and "chair I" forms, respectively, among which the boat form is dominant. All the six DB18C6-(H(2)O)(n) clusters with n = 1-4 have a boat conformation in the DB18C6 part. The water molecules form a variety of hydration networks in the boat-DB18C6 cavity. In DB18C6-(H(2)O)(1), a water molecule forms the bidentate hydrogen bond with the O atoms adjacent to the benzene rings. In this cluster, the water molecule is preferentially hydrogen bonded from the bottom of boat-DB18C6. In the larger clusters, the hydration networks are developed on the basis of the DB18C6-(H(2)O)(1) cluster.

Resonant two-photon ionization and ab initio conformational analysis of haloethyl benzenes (PhCH2CH2X,X=Cl,F)

The S(1) <-- S(0) transitions of the gaseous (2-fluoroethyl)-benzene (FEB) and (2-chloroethyl)-benzene (CEB) have been investigated using a combination of two-color resonant two-photon ionization and UV-UV hole burning spectroscopy. Both anti and gauche conformers have been identified on the basis of rotational band contour analysis supported by ab initio calculations on the ground and electronically excited states. The gauche origin band of FEB at 37,673 cm(-1) is redshifted 50 cm(-1) relative to the corresponding anti origin, while CEB origin bands overlap at 37,646 cm(-1). Relative conformational stability and populations in the jet have been estimated for both molecules, based on the intensity ratio of S(1) <-- S(0) band origin transitions. These are compared with a range of related molecules with the structural motif PhCH(2)CH(2)X (X=CH(3),CH(2)CH(3),NH(2),OH,COOH,CCH,CN). Theory and experimental results for FEB and CEB show repulsive interactions between the halogen substituents and the pi cloud of the phenyl rings destabilizing the gauche conformers, but the preference for the anti conformers is relatively modest. The gauche conformer origins show very different hybrid character: FEB is largely b type, while CEB is an ac hybrid in keeping with theoretically computed TM "rotations" (theta(elec)) of -7 degrees and -56 degrees , respectively. This difference is attributed largely to rotation of the side chain in opposite directions about the C(1)C(alpha) bond. Spectra of FEB(H(2)O) and CEB(H(2)O) single water clusters show evidence of an anti conformation in the host molecule.