R2PI Spectra Research Articles

Spectroscopic Characterization of Hydrogen-Bonded 2,7-Diazaindole Water Complex Isolated in the Gas Phase.

We present a systematic experimental analysis of the 1:1 complex of 2,7-diazaindole (27DAI) with water in the gas phase. The complex was characterized by using two-color-resonant two-photon ionization (R2PI), laser-induced fluorescence (LIF), single vibronic level fluorescence (SVLF), and photoionization efficiency (PIE) spectroscopic methods. The 000 band of the S1←S0 electronic transition of the 27DAI-H2O complex was observed at 33,074 cm-1, largely red-shifted by 836 cm-1 compared to that of the bare 27DAI. From the R2PI spectrum, the detected modes at 141 (ν'Tx), 169 (ν'Ty), and 194 (ν'Ry) cm-1 were identified as the internal motions of the H2O molecule in the complex. However, these modes were detected at 115 (ν″Tx), 152 (ν″Ty), and 190 (ν″Ry) cm-1 in the ground state, which suggested a stronger hydrogen bonding interaction in the photo-excited state. The structural determination was aided by the detection of νNH and νOH values in the ground and excited state complexes using the FDIR and IDIR spectroscopies. The detection of νNH at 3414 and νOH at 3447 cm-1 in 27DAI-H2O has shown an excellent correlation with the most stable structure consisting of N(1)-H···O and OH···N(7) hydrogen-bonded bridging water molecule in the ground state. The structure of the complex in the electronic excited state (S1) was confirmed by the corresponding bands at 3210 (νNH) and 3265 cm-1 (νOH). The IR-UV hole-burning spectroscopy confirmed the presence of only one isomer in the molecular beam. The ionization energy (IE) of the 27DAI-H2O complex was obtained as 8.789 ± 0.002 eV, which was significantly higher than the 7AI-H2O complex. The higher IE values of N-rich molecules suggest a higher resistivity of such molecules against photodamage. The obtained structure of the 27DAI-H2O complex has explicitly shown the formation of a cyclic one-solvent bridge incorporating N(1)-H···O and O-H···N(7) hydrogen bonds upon microsolvation. The lower excitation and higher ionization energies of the 27DAI-H2O complex compared to 7AI-H2O established higher stabilization of N-rich molecules. The solvent clusters forming a linear bridge between the hydrogen/proton acceptor and donor sites in the complex can be considered as a stepping stone to investigate the photoinduced deactivation mechanisms in nitrogen containing biologically relevant molecules.

Bond dissociation energies of diatomic transition metal nitrides.

Resonant two-photon ionization (R2PI) spectroscopy has been used to measure the bond dissociation energies (BDEs) of the diatomic transition metal nitrides ScN, TiN, YN, MoN, RuN, RhN, HfN, OsN, and IrN. Of these, the BDEs of only TiN and HfN had been previously measured. Due to the many ways electrons can be distributed among the d orbitals, these molecules possess an extremely high density of electronic states near the ground separated atom limit. Spin-orbit and nonadiabatic interactions couple these states quite effectively, so that the molecules readily find a path to dissociation when excited above the ground separated atom limit. The result is a sharp drop in ion signal in the R2PI spectrum when the molecule is excited above this limit, allowing the BDE to be readily measured. Using this method, the values D0(ScN) = 3.905(29) eV, D0(TiN) = 5.000(19) eV, D0(YN) = 4.125(24) eV, D0(MoN) = 5.220(4) eV, D0(RuN) = 4.905(3) eV, D0(RhN) = 3.659(32) eV, D0(HfN) = 5.374(4) eV, D0(OsN) = 5.732(3) eV, and D0(IrN) = 5.115(4) eV are obtained. To support the experimental findings, ab initio coupled-cluster calculations extrapolated to the complete basis set limit (CBS) were performed. With a semiempirical correction for spin-orbit effects, these coupled-cluster single double triple-CBS calculations give a mean absolute deviation from the experimental BDE values of 0.20eV. A discussion of the periodic trends, summaries of previous work, and comparisons to isoelectronic species is also provided.

Spectroscopic and theoretical studies of jet‐cooled 3‐cyanoindole ammonia clusters in the gas phase

AbstractThe structural information of 3‐cyanoindole ammonia cluster, 3CI(NH3)1, in molecular beams was investigated in the gas phase. We obtained UV–UV hole‐burning spectrum (UV–UV HB) to determine the number of conformers present in the molecular beams and examine their spectral features. As a result, single conformer of 3CI(NH3)1 was observed in the mass‐selected one‐color resonant two‐photon ionization (1C‐R2PI) spectroscopy. The origin band of 3CI(NH3)1 in the R2PI spectrum is redshifted by 122 and 71 cm−1 compared to those of the 3CI monomer and 3CI 1:1 water cluster, 3CI(H2O)1, respectively. The fully optimized geometries, relative energies, transition energies, binding energies, and electrostatic potential (ESP) of the 3CI monomer, 3CI(H2O)1, and 3CI(NH3)1 clusters were calculated at the B3LYP and TD‐B3LYP methods with the 6‐311++(d,p) basis set. The observed spectra are compared with the predictions of time‐dependent density functional theory calculations.

Polarization dependence of the resonant two‐photon ionization spectra of jet‐cooled (1R,2R)‐(−)‐pseudoephedrine

AbstractThe polarization dependence of the resonant two‐photon ionization (R2PI) spectra of jet‐cooled (1R,2R)‐(−)‐pseudoephedrine was investigated near the origin bands of three conformers. To obtain the difference R2PI spectra of the origin bands, R2PI signals produced by circularly polarized light (CP) were subtracted from those produced by linearly polarized light (LP), which revealed positive values for the P and R branches of the origin bands and negative values for the Q branch. To understand the different polarization behaviors of the P, Q, and R branches, expressions were derived to predict the transition probabilities of symmetric top molecules in a parallel transition. The transition probabilities varied depending on the direction of the total angular momentum J with respect to the light propagation vector. The different polarization behaviors of the rotational branches may arise from the differences in the angular distributions of the J vectors that are preferentially excited by LP and CP.

A compact and cost-effective laser desorption source for molecular beam generation: comparison with simulations

The development of laser-desorption jet-cooling has enabled the study of low vapor pressure molecules in the gas phase. Here, we design and assemble a unique compact laser-desorption source comprising simple parts for sample movement and desorption and attach it to a home-built time-of-flight mass spectrometer for spectroscopic studies of jet-cooled biomolecules. Modeling and visualization of the molecular beam (MB) by the direct simulation Monte Carlo method provides spatial evolution and global features of the flow and new insights into source operation. The system is validated by determining operational parameters from the simulation and comparing them with the resulting resonance-enhanced two-photon ionization (R2PI) signal of tryptamine. The signal measurements in the origin band transitions region provide an R2PI spectrum resembling that previously acquired from heated samples, indicating seven conformers in the MB. Due to the potential of this source, we expect that it will reveal its feasibility through coupling to other systems and various spectroscopies.

Locating Cytosine Conical Intersections by Laser Experiments and Ab Initio Calculations.

The decay mechanism of S0 → S1 excited cytosine (Cyt) and the effect of substitution are studied combining jet-cooled spectroscopy (nanosecond resonant two-photon ionization (R2PI) and picosecond lifetime measurements) with CASPT2//CASSCF computations for eight derivatives. For Cyt and five derivatives substituted at N1, C5, and C6, rapid internal conversion sets in at 250-1200 cm-1 above the 000 bands. The break-off in the spectra correlates with the calculated barriers toward the "C5-C6 twist" conical intersection, which unambiguously establishes the decay mechanism at low S1 state vibrational energies. The barriers increase with substituents that stabilize the charge shifts at C4, C5, and C6 following (1ππ*) excitation. The R2PI spectra of the clamped derivatives 5,6-trimethyleneCyt (TMCyt) and 1-methyl-TMCyt (1M-TMCyt), which decay along an N3 out-of-plane coordinate, extend up to +3500 and +4500 cm-1.

R2PI and MATI spectroscopy of 3-fluoro-5-methylanisole: Combined effect of meta-substituents on molecular conformation.

The resonance-enhanced two-photon ionization (R2PI) and mass-analyzed threshold ionization (MATI) spectra of 3-fluoro-5-methylanisole (3F5MA) were recorded to explore the conformers arising from the rotation of meta-methyl group (m-CH3) and methoxy group (OCH3), namely the staggered (s)/eclipsed (e)-cis/trans 3F5MA. The theoretical calculations predicted that the stable conformer of cis 3F5MA is staggered in the S0 and S1 states, but eclipsed in the D0 state. While for trans 3F5MA, the staggered conformer is stable only in the S1 state and the eclipsed one is stable in the S0 and D0 states. The first electronic excitation energies (E1s) of cis and trans 3F5MA were determined to be 36,709 ± 3 and 36,615 ± 3 cm-1 by the R2PI spectroscopy. Correspondingly, by the MATI spectroscopy, the adiabatic ionization energies (IEs) were measured to be 66,908 ± 5 and 66,692 ± 5 cm-1. Compared with the cis 3F5MA, more low-frequency vibronic bands assigned to m-CH3 torsions are observed for the trans conformer in the R2PI spectrum, supporting the theoretically predicated e-trans → s-trans isomerization upon excitation. The MATI spectra of the cis and trans conformers are similar. Most of the observed cationic bands are related to the m-CH3 torsion modes, revealing the s-cis → e-cis and s-trans → e-trans isomerizations upon the D0 ← S1 ionization. By comparing with several analogues, it is found that the s/e conformational preference in each electronic state is mainly influenced by OCH3 group instead of the F atom. The combined effects of meta-substituents on molecular conformation and transition energies are discussed in detail. A kind of additivity of meta-substituent effects on s/e preference is qualitatively true for the S0 and S1 states but false for the D0 state. This is different from that of ortho-substituent effect, which is previously reported that the additivity of ortho-substituent effects on methyl rotation barriers in 2-fluoro-6-chlorotoluene is applicable to all the three electronic states while the additivity of meta-substituent effect on cis/trans preference is found to be true in all three electronic states of 3F5MA.

Planarizing cytosine: The S1 state structure, vibrations, and nonradiative dynamics of jet-cooled 5,6-trimethylenecytosine.

We measure the S0 → S1 spectrum and time-resolved S1 state nonradiative dynamics of the "clamped" cytosine derivative 5,6-trimethylenecytosine (TMCyt) in a supersonic jet, using two-color resonant two-photon ionization (R2PI), UV/UV holeburning, and ns time-resolved pump/delayed ionization. The experiments are complemented with spin-component scaled second-order approximate coupled cluster (SCS-CC2), time-dependent density functional theory, and multi-state second-order perturbation-theory (MS-CASPT2) ab initio calculations. While the R2PI spectrum of cytosine breaks off ∼500 cm-1 above its 000 band, that of TMCyt extends up to +4400 cm-1 higher, with over a hundred resolved vibronic bands. Thus, clamping the cytosine C5-C6 bond allows us to explore the S1 state vibrations and S0 → S1 geometry changes in detail. The TMCyt S1 state out-of-plane vibrations ν1', ν3', and ν5' lie below 420 cm-1, and the in-plane ν11', ν12', and ν23' vibrational fundamentals appear at 450, 470, and 944 cm-1. S0 → S1 vibronic simulations based on SCS-CC2 calculations agree well with experiment if the calculated ν1', ν3', and ν5' frequencies are reduced by a factor of 2-3. MS-CASPT2 calculations predict that the ethylene-type S1 ⇝ S0 conical intersection (CI) increases from +366 cm-1 in cytosine to >6000 cm-1 in TMCyt, explaining the long lifetime and extended S0 → S1 spectrum. The lowest-energy S1 ⇝ S0 CI of TMCyt is the "amino out-of-plane" (OPX) intersection, calculated at +4190 cm-1. The experimental S1 ⇝ S0 internal conversion rate constant at the S1(v'=0) level is kIC=0.98-2.2⋅108 s-1, which is ∼10 times smaller than in 1-methylcytosine and cytosine. The S1(v'=0) level relaxes into the T1(3ππ*) state by intersystem crossing with kISC=0.41-1.6⋅108 s-1. The T1 state energy is measured to lie 24 580±560 cm-1 above the S0 state. The S1(v'=0) lifetime is τ=2.9 ns, resulting in an estimated fluorescence quantum yield of Φfl=24%. Intense two-color R2PI spectra of the TMCyt amino-enol tautomers appear above 36 000 cm-1. A sharp S1 ionization threshold is observed for amino-keto TMCyt, yielding an adiabatic ionization energy of 8.114±0.002 eV.

REMPI and MATI spectroscopy of cis and trans 4-chloro-3-fluoroanisole

The Resonance-enhanced two-photon ionization (R2PI) and Mass-analyzed threshold ionization (MATI) spectra of 4-chloro-3-fluoroanisole (4Cl3FA) were recorded in detail. The experimental and calculated results revealed that cis and trans 4Cl3FA are the only two stable conformers in each of the S0, S1 and D0 states. The first electronic excitation energies (E1’s) of cis and trans35Cl-4Cl3FA were measured to be 35,326±3 and 35,443±3cm−1 by the R2PI spectroscopy and the corresponding adiabatic ionization energies (IEs) were determined to be 67,330±5 and 67,591±5cm−1 by the MATI spectroscopy. The E1’s and IEs of cis and trans37Cl-4Cl3FA were also obtained and found to be identical to that of the 35Cl-4Cl3FA conformers. Compared with the trans 4Cl3FA, the cis 4Cl3FA is more stable in the S0, S1 and D0 states. The vibrational bands observed in the R2PI and MATI spectra are related to the vibrations involving the in-plane deformation of the benzene ring, the CH3 torsion and the OCH3 inversion. The substituent effects on the molecular structures, transition energies and vibrations of 4Cl3FA are discussed.

REMPI and MATI spectroscopy of cis and trans 3-chloro-N-methylaniline

The Resonance-enhanced two-photon ionization (R2PI) and Mass-analyzed threshold ionization (MATI) spectra of 3-Chloro-N-methylaniline (3ClNMA) were recorded in detail. The experimental and calculated results revealed that cis and trans 3ClNMA are the only two stable conformers for each of the S0, S1 and D0 states. The first electronic excitation energies (E1s) of cis and trans35Cl-3ClNMA were measured to be 33,003±3 and 32,886±3cm−1 by R2PI spectroscopy. The adiabatic ionization energies (IEs) of cis and trans35Cl-3ClNMA were determined to be 61,531±5 and 61,625±5cm−1 by MATI spectroscopy. The E1s and IEs of cis and trans37Cl-3ClNMA were also obtained and found to be identical to that of the 35Cl-3ClNMA rotamers. Compared with the trans rotamer, the cis rotamer is less stable by 50cm−1 in the S1 state, but more stable by 67 and 161cm−1 in S0 and D0 states. Most of the active vibration modes observed in the R2PI and MATI spectra are in-plane ring deformation and substituent-sensitive modes. Detailed analysis reveals a correlation between the typical ring vibration frequencies and the relative stability of the two rotamers, suggesting that the conformation effect on the ring rigidity plays a key role in determining the relative stability of the conformers. The ab initio and density functional theory (DFT) calculations were performed to analyze the substitution and conformation effects on the properties of 3ClNMA rotamers.

Electronic and vibrational spectroscopic studies of jet-cooled 5-cyanoindole and its water clusters, 5CI–(H2O)n, (n = 0–2)

Mass-selected resonant two-photon ionization (R2PI) and UV–UV hole-burning, and infrared-dip spectra of 5-cyanoindole (5CI) and its water clusters, 5CI–(H2O)n (n=1 and 2) were measured. Although, the structures of 5CI–(H2O)1–2 are similar to those of 3CI–(H2O)1–2, the photofragmentation behaviors of the two systems are quite different due to the La–Lb state energy lowering and higher binding energies of 5CI–(H2O)1–2 compared to those of 3CI–(H2O)1–2. Especially for the case of 5CI–(H2O)2 cluster, shortening excited-state lifetime of 5CI–(H2O)2 causes the broad background in the R2PI spectrum of 5CI–(H2O)2.

Chlorine Para-Substitution of 1-Phenylethanol: Resonant Photoionization Spectroscopy and Quantum Chemical Calculations of Hydrated and Diastereomeric Complexes.

The conformational landscape of (S)-1-(4-chlorophenyl)ethanol, its monohydrated complex, and its diastereomeric adducts with R- and S-butan-2-ol, have been investigated by resonant two-photon ionization (R2PI) spectroscopy coupled with time-of-flight mass spectrometry. Theoretical calculations at the D-B3LYP/6-31++G** level of theory have been performed to assist in the interpretation of the spectra and in the assignment of the structures. The R2PI spectra and the predicted structures have been compared with those obtained on the analogous non-halogenated and fluorinated systems, i.e., (R)-1-phenylethanol and (S)-1-(4-fluorophenyl)ethanol, respectively. It appears that the presence of chlorine atom in the para position of the aromatic ring does not influence the overall geometry of bare molecule and its complexes with respect to the non-halogenated analogous systems. Anyway, it affects the electron density in the π system, and in turn the strength of OH···π and CH···π interactions. A spectral chiral discrimination is evident from the R2PI spectra of the diastereomeric adducts of (S)-1-(4-chlorophenyl)ethanol with the two enantiomers of butan-2-ol.

Resonant two-photon ionization spectroscopy of Al atoms and dimers solvated in helium nanodroplets.

Resonant two-photon ionization (R2PI) spectroscopy has been applied to investigate the solvation of Al atoms in helium droplets. The R2PI spectra reveal vibrational progressions that can be attributed to Al-He(n) vibrations. It is found that small helium droplets have very little chance to pick up an aluminum atom after collision. However, the pick-up probability increases with the size of the helium droplets. The absorption band that is measured by monitoring the ions on the mass of the Al dimer is found to be very little shifted with respect to the Al monomer band (∼400 cm(-1)). However, using the same laser wavelength, we were unable to detect any Al(n) photoion with n larger than two.

Conformational structures of 3-cyanoindole-(H2O)n (n = 0–2): Franck–Condon simulations

Franck–Condon (FC) simulations were applied to 3-cyanoindole (3CI) and its water clusters, 3CI-(H2O)n (n=1 and 2), for the first time to determine their molecular structures using mass-selected resonant two-photon ionization (R2PI) and UV–UV hole-burning (UVHB) spectroscopy. R2PI and UVHB spectra were compared with the results of Franck–Condon (FC) simulations that can determine the change in a molecular geometry of 3CI-(H2O)n (n=0–2) on electronic excitation. It was found that the most stable structure for each 3CI-(H2O)n (n=0–2) was identified and their excited state geometries were little influenced upon excitation.

Reactivity of Iron Atoms at Low Temperature

We have studied the reactions of iron atoms and clusters with oxygen, acetylene, and water molecules in superfluid He droplets at T = 0.37 K. For all systems, the formation of weakly bound adducts was found, but the insertion reaction of iron into existing molecular bonds could not be observed. The formation of FeOH2 and FeC2H2 complexes was evidenced by mass spectrometry. However, it was found that the reaction of iron atoms with oxygen molecules under similar conditions leads to the stabilization of an intermediate reaction product, the weakly bound linear FeOO adduct, which undergoes complete dissociation upon electron impact ionization. All reactions observed are not expected to proceed in the gas phase. The R2PI spectrum of the y5D4 0 ← a5D4 atomic transition of Fe solvated in helium droplets was recorded. A relatively small blue shift of ∼ 120 cm(-1) with respect to the gas phase position was found.

Role of Ring-Constrained γ-Amino Acid Residues in α/γ-Peptide Folding: Single-Conformation UV and IR Spectroscopy

The capped α/γ-peptide foldamers Ac-γACHC-Ala-NH-benzyl (γα) and Ac-Ala-γACHC-NH-benzyl (αγ) were studied in the gas phase under jet-cooled conditions using single-conformation spectroscopy. These molecules serve as models for local segments of larger heterogeneous 1:1 α/γ-peptides that have recently been synthesized and shown to form a 12-helix composed of repeating C12 H-bonded rings both in crystalline form and in solution [Guo, L.; et al. J. Am. Chem. Soc. 2009, 131, 16018]. The γα and αγ peptide subunits are structurally constrained at the Cβ-Cγ bond of the γ-residue with a cis-cyclohexyl ring and by an ethyl group at the Cα position. These triamides are the minimum length necessary for the formation of the C12 H-bond. Resonant two-photon ionization (R2PI) provides ultraviolet spectra that have contributions from all conformational isomers, while IR-UV hole-burning (IR-UV HB) and resonant ion-dip infrared (RIDIR) spectroscopies are used to record single-conformation UV and IR spectra, respectively. Four and six conformers are identified in the R2PI spectra of the γα and αγ peptides, respectively. RIDIR spectra in the NH stretch, amide I (C═O stretch), and amide II (NH bend) regions are compared with the predictions of density functional theory (DFT) calculations at the M05-2X/6-31+G* level, leading to definite assignments for the H-bonding architectures of the conformers. While the C12 H-bond is present in both γα and αγ, C9 rings are more prevalent, with seven of ten conformers incorporating a C9 H-bond involving in the γ-residue. Nevertheless, comparison of the assigned structures of gas-phase γα and αγ with the crystal structures for γα and larger α/γ-peptides reveals that the constrained γ-peptide backbone formed by the C9 ring is structurally similar to that formed by the larger C12 ring present in the 12-helix. These results confirm that the ACHC/ethyl constrained γ-residue is structurally preorganized to play a significant role in promoting C12 H-bond formation in larger α/γ-peptides.

π-Stacking, C–H/π, and Halogen Bonding Interactions in Bromobenzene and Mixed Bromobenzene–Benzene Clusters

Noncovalent interactions play an important role in many chemical and biochemical processes. Building upon our recent study of the homoclusters of chlorobenzene, where π-π stacking and CH/π interactions were identified as the most important binding motifs, in this work we present a study of bromobenzene (PhBr) and mixed bromobenzene-benzene clusters. Electronic spectra in the region of the PhBr monomer S0-S1 (ππ*) transition were obtained using resonant two-photon ionization (R2PI) methods combined with time-of-flight mass analysis. As previously found for related systems, the PhBr cluster spectra show a broad feature whose center is red-shifted from the monomer absorption, and electronic structure calculations indicate the presence of multiple isomers and Franck-Condon activity in low-frequency intermolecular modes. Calculations at the M06-2X/aug-cc-pVDZ level find in total eight minimum energy structures for the PhBr dimer: four π-stacked structures differing in the relative orientation of the Br atoms (denoted D1-D4), one T-shaped structure (D5), and three halogen bonded structures (D6-D8). The calculated binding energies of these complexes, corrected for basis set superposition error (BSSE) and zero-point energy (ZPE), are in the range of -6 to -24 kJ/mol. Time-dependent density functional theory (TDDFT) calculations predict that these isomers absorb over a range that is roughly consistent with the breadth of the experimental spectrum. To examine the influence of dipole-dipole interaction, R2PI spectra were also obtained for the mixed PhBr···benzene dimer, where the spectral congestion is reduced and clear vibrational structure is observed. This structure is well-simulated by Franck-Condon calculations that incorporate the lowest frequency intermolecular modes. Calculations find four minimum energy structures for the mixed dimer and predict that the binding energy of the global minimum is reduced by ~30% relative to the global minimum PhBr dimer structure.

Excited-State Structure and Dynamics of Keto–Amino Cytosine: The 1ππ* State Is Nonplanar and Its Radiationless Decay Is Not Ultrafast

We have measured the mass- and tautomer-specific S0 → S1 vibronic spectra and S1 state lifetimes of the keto–amino tautomer of cytosine cooled in supersonic jets, using two-color resonant two-photon ionization (R2PI) spectroscopy at 0.05 cm(–1) resolution. The rotational contours of the 0(0)(0) band and nine vibronic bands up to +437 cm(–1) are polarized in the pyrimidinone plane, proving that the electronic excitation is to a 1ππ* state. All vibronic excitations up to +437 cm(–1) are overtone and combination bands of the low-frequency out-of-plane ν1′ (butterfly), ν2′ (boat), and ν3′ (H–N–C6–H twist) vibrations. UV vibronic spectrum simulations based on approximate second-order coupled-cluster (CC2) calculations of the ground and 1ππ* states are in good agreement with the experimental R2PI spectrum, but only if the calculated ν1′ and ν2′ frequencies are reduced by a factor of 4 and anharmonicity is included. Together with the high intensity of the ν1′ and ν2′ overtone vibronic excitations, this implies that the 1ππ* potential energy surface is much softer and much more anharmonic in the out-of-plane directions than predicted by the CC2 method. The 1ππ* state lifetime is determined from the Lorentzian line broadening necessary to reproduce the rotational band contours: at the 0(0)(0) band it is τ = 44 ps, remains at τ = 35–45 ps up to +205 cm(–1), and decreases to 25–30 ps up to +437 cm(–1). These lifetimes are 20–40 times longer than the 0.5–1.5 ps lifetimes previously measured with femtosecond pump–probe techniques at higher vibrational energies (1500–3800 cm(–1)). Thus, the nonradiative relaxation rate of keto–amino cytosine close to the 1ππ* state minimum is k(nr) 2.5 × 10(10) s(–1), much smaller than at higher energies. An additional nonradiative decay channel opens at +500 cm(–1) excess energy. Since high overtone bands of ν1′ and ν2′ are observed in the R2PI spectrum but only a single weak 2ν3′ band, we propose that ν3′ is a promoting mode for nonradiative decay, consistent with the observation that the ν3′ normal-mode eigenvector points toward the “C6-puckered” conical intersection geometry.

Vibronic and cation spectroscopy of structural isomers p- and m-diaminobenzene and the amino substitution effect

► We report the R2PI and MATI spectra of p -diaminobenzene and m -diaminobenzene. ► The first electronic transition and ionization energies are precisely measured. ► The data help us to learn the amino substitution effects on molecular properties. We applied the resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibronic and cation spectra of p -diaminobenzene (PDAB) and m -diaminobenzene (MDAB). The band origins of the S 1 ← S 0 electronic transition of PDAB and MDAB were found to be 29 805 ± 2 and 33 395 ± 2 cm −1 , whereas the adiabatic ionization energies were determined to be 54 624 ± 5 and 58 321 ± 5 cm −1 , respectively. Most of the observed active vibrations of these two structural isomers in the electronically excited S 1 and cationic ground D 0 states result from in-plane ring deformation and bending vibrations. Comparing the present data with those of aniline suggests that the interaction of the amino substituent and aromatic ring in the upper (S 1 or D 0 ) state is stronger than that in the S 0 state. In addition, the frequency of observed active vibrations may depend on the nature, relative location, vibrational pattern, and the extent of the amino group participated in the normal vibration.

Out-of-Plane Low-Frequency Vibrations and Nonradiative Decay in the 1ππ* State of Jet-Cooled 5-Methylcytosine

We investigate the UV vibronic spectrum and excited-state nonradiative processes of jet-cooled 5-methylcytosine (5MCyt) using two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm(–1) resolution. In contrast to cytosine, which shows only five bands above its electronic origin, the lowest electronic transition of 5MCyt exhibits about 25 low-frequency vibronic bands that extend to 0(0)(0) + 450 cm(–1), allowing to extract detailed information on the excited-state electronic and nuclear structure. Most bands are overtones and combinations of the out-of-plane vibrations ν'(1), ν'(2), and ν'(3). Their large intensities reflect butterfly-, boat-, and twist-deformations of the 5MCyt framework upon electronic excitation. From the rotational contours of the 0(0)(0), 1(0)(2), 2(0)(2), and 3(0)(2) bands, the transition is found to be polarized along the in-plane a/b axes, characteristic of a (1)ππ* transition. Approximate second-order coupled-cluster (CC2) and time-dependent B3LYP calculations both predict that 5MCyt undergoes an out-of-plane deformation in its (1)ππ* (S(2)) state but both methods overestimate the out-of-plane ν'(1), ν'(2), and ν'(3) vibrational frequencies by a factor of 3–5. The TD-B3LYP (1)ππ* transition dipole moment direction is 10%:90% a:b, in good agreement with experiment. From the Lorentzian line shape contributions needed to fit the rotational contours, a lower limit to the 5MCyt (1)ππ* state lifetime at the 0(0)(0), 1(0)(2), 2(0)(2), and 3(0)(2) bands is determined as τ ≥ 30 ps. These values are in stark contrast to the ultrafast (picosecond) lifetimes measured for jet-cooled cytosine by femtosecond pump–probe techniques. They also confirm the observation from the R2PI spectrum that 5-methylation of cytosine increases its excited-state lifetime. The higher out-of-plane overtone and combination bands disappear from the spectrum by ~460 cm(–1), signaling the onset of lifetimes τ < 0.5 ps, induced by excitation of the ν'(1), ν'(2), and ν'(3) modes. Above 460 cm(–1) the in-plane fundamentals 9(0)(1), 10(0)(1), and 17(0)(1) are weakly observed, implying that the in-plane vibrations couple weakly to the out-of-plane modes and have lifetimes τ > 3 ps up to 720 cm(–1) excess vibrational energy.