Isolated Molecule Conditions Research Articles

Comparing molecular photofragmentation dynamics in the gas and liquid phases

This article explores the extent to which insights gleaned from detailed studies of molecular photodissociations in the gas phase (i.e. under isolated molecule conditions) can inform our understanding of the corresponding photofragmentation processes in solution. Systems selected for comparison include a thiophenol (p-methylthiophenol), a thioanisole (p-methylthioanisole) and phenol, in vacuum and in cyclohexane solution. UV excitation in the gas phase results in RX-Y (X = O, S; Y = H, CH3) bond fission in all cases, but over timescales that vary by ~4 orders of magnitude - all of which behaviours can be rationalised on the basis of the relevant bound and dissociative excited state potential energy surfaces (PESs) accessed by UV photoexcitation, and of the conical intersections that facilitate radiationless transfer between these PESs. Time-resolved UV pump-broadband UV/visible probe and/or UV pump-broadband IR probe studies of the corresponding systems in cyclohexane solution reveal additional processes that are unique to the condensed phase. Thus, for example, the data clearly reveal evidence of (i) vibrational relaxation of the photoexcited molecules prior to their dissociation and of the radical fragments formed upon X-Y bond fission, and (ii) geminate recombination of the RX and Y products (leading to reformation of the ground state parent and/or isomeric adducts). Nonetheless, the data also show that, in each case, the characteristics (and the timescale) of the initial bond fission process that occurs under isolated molecule conditions are barely changed by the presence of a weakly interacting solvent like cyclohexane. These condensed phase studies are then extended to an ether analogue of phenol (allyl phenyl ether), wherein UV photo-induced RO-allyl bond fission constitutes the first step of a photo-Claisen rearrangement.

Importance of London dispersion effects for the packing of molecular crystals: a case study for intramolecular stacking in a bis-thiophene derivative

The influence of London dispersion effects in density functional theory for the computation of molecular crystals is investigated. The structure of dithienobicyclo[4.4.1]-undeca-3,8-dien-11-one ethylene glycol ketal (Resvan) that has been studied previously under isolated molecule conditions is considered as an example. The intramolecular stacking of the two thiophene rings is strongly influenced by non-covalent interactions. Previous investigations revealed significant deviations between computed molecular and experimentally observed crystal sulfur-sulfur distances. The influence of the crystal environment on this distance is estimated by calculating potential energy curves for the "gas phase" and in the crystal using the periodic plane-wave code VASP. With three common dispersion corrected (DFT-D3) density functionals (PBE, revPBE and PBEsol), an average shortening of the sulfur-sulfur distance by 0.10 A ("packing" effect) is computed which leads to a corrected "experimental" gas-phase value of R(S-S) = 4.19 A. This value is in good agreement with the results of reliable quantum chemical methods and serves as a challenging benchmark for electronic structure methods. Overall the crystal environment leads to a widening of the molecular structure due to intermolecular dispersion interaction. This is opposite to uncorrected DFT that suggest a compression due to Pauli exchange repulsion. In the crystal the intra- and inter-molecular dispersion effects are large but of opposite sign and their effect almost cancels out so that in this example dispersion-uncorrected DFT yields reasonable solid state structures. This is different for the computation of the lattice energy for which reasonable values are obtained only at the DFT-D3 level.

Laser Spectroscopy of Conformationally Constrained α/β-Peptides: Ac-ACPC-Phe-NHMe and Ac-Phe-ACPC-NHMe

Single-conformation ultraviolet and infrared spectra have been recorded under the isolated molecule conditions of a supersonic expansion for three conformationally constrained alpha/beta-peptides, Ac-L-Phe-ACPC-NHMe (alpha(L)beta(ACPC)), Ac-ACPC-L-Phe-NHMe (beta(ACPC)alpha(L)), and Ac-ACPC-D-Phe-NHMe (beta(ACPC)alpha(D)). These three molecules are close analogues of the hAla-containing alpha/beta-peptide counterparts Ac-L-Phe-beta(3)-hAla-NHMe, Ac-beta(3)-hAla-L-Phe-NHMe, and Ac-beta(3)-hAla-D-Phe-NHMe, which have been studied recently by James et al. (J. Am. Chem. Soc. 2009, 131, 6574). Incorporation of the beta-amino acid trans-2-aminocyclopentanecarboxylic acid (ACPC) constrains the beta-peptide backbone via the cyclopentane ring, producing clear changes in the conformational preferences relative to the unconstrained analogues. The conformational control is manifested most obviously in the complete absence of C6 H-bonded rings, which were dominant in the unconstrained alpha/beta-peptides. The most stable C6 ring structure (C6a) in the absence of the ACPC ring cannot be formed in its presence, while a secondary C6 ring (C6b) has its energy destabilized by approximately 20 kJ/mol. In alpha(L)beta(ACPC), the preference for C5 structures in the N-terminal position, combined with the strong preference for C8 structures in the beta-peptide subunit, leads to the observation of two C5/C8 bifurcated double ring conformers. Both C8/C7 sequential double rings and C11 single rings are observed in beta(ACPC)alpha(L) and beta(ACPC)alpha(D). Here, the ACPC ring selectively stabilizes the C8a ring over other possible C8 structures. Finally, the combined evidence from IR and UV spectra lead to tentative assignments for diastereomeric pairs, exhibiting small but understandable shifts in the IR and UV spectra induced by the change in chirality at the alpha-peptide chiral center.

Intramolecular Amide Stacking and Its Competition with Hydrogen Bonding in a Small Foldamer

Attractive interactions between two carboxamide groups in a "stacked" geometry are explored under isolated molecule conditions. Infrared spectra of single conformations of a small gamma-peptide, Ac-gamma(2)-hPhe-NHMe, reveal the presence of a conformation in which the two amide planes are approximately parallel with the amide dipoles in an antialigned orientation. This stacked conformation is energetically comparable to conformations that contain an intramolecular amide-amide H-bond. Amide stacking interactions can compete with H-bonding in circumstances where the amide groups can be brought into a stacking configuration with minimal strain, opening the way for its use in the design of future foldamer structures.

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.

Absence of dual fluorescence with 4-(dimethylamino) phenylacetylene. A comparison between experimental results and theoretical predictions

From ab initio electronic structure calculations it has been predicted that 4-(dimethylamino)phenylacetylene (DACET) will undergo an exothermic intramolecular charge transfer (ICT) reaction with dual fluorescence even under isolated-molecule conditions. Photostationary and time-resolved fluorescence measurements reveal however that an ICT reaction does not occur with DACET under any condition of solvent polarity or temperature studied. DACET is similar to 4-(methyl)amino-benzonitrile, which shows only fluorescence from a locally excited state, but is clearly different from the dual fluorescent 4-(dimethyl)aminobenzonitrile (DMABN). The energy gap Δ E( S 1, S 2) of DACET is larger than that of DMABN, supporting the importance of this gap in these ICT reactions.

Mechanism of photoinduced intramolecular charge transfer in aminobenzethynes: an ab initio study

Ab initio electronic structure calculations were performed to characterize the charge transfer (CT) process in benzethyne (BE), 4-aminobenzethyne (ABE) and 4-dimethylamino-benzethyne (DMABE). The Hartree-Pock (HF), configuration interaction scheme (CIS ), complete active space self-consistent field (CASSCF) and CASPT2 (multiconfigurational second-order perturbation theory taking CASSCF function as a reference) methods were employed. Geometry optimization of the CT state at the CIS level predicts a planar configuration with a trans bent ethyne group to be the lowest excited singlet state in all systems. The significant stabilization of the CT state in the BE series can be understood as arising from the rehybridization of the carbon atoms of the ethyne group on transfer of an electron. The calculation of the relative energy of the locally excited state and the relaxed CT state using the CASPT2 method reveals that the intramolecular CT process is exothermic in ABEs under isolated molecule conditions, in contrast with the much studied aminohenzonitriles, where interaction with a polar solvent is required for sufficient stabilization of the CT state. Therefore BE and its amino derivatives potentially offer unique opportunities for the investigation of photoinduced intramolecular CT in isolated molecules.

Translational energy release following multiphoton dissociation of organometallics

We have measured translational energy distributions for state selected metal atoms produced by multiphoton dissociation (MPD) of organometallics in supersonic nozzle beams under isolated molecule conditions. Ground state chromium atoms liberated by 361 nm MPD of Cr(CO) 6 are produced at ≈ 2200 K. This shows that the only significant MPD pathway must be a net three photon process. 7S 3 tungsten atoms produced from W(CO) 6 are produced in a ≈ 4000 K Maxwell-Boltzmann distribution. The results of these experiments are discussed in terms of energetics and their implications for laser driven particle and thin film formation.

Comparisons between the reactivity of chlorine dioxide in the gas phase and water solution

The thermal and photochemical reactivity of the symmetric isomer of chlorine dioxide, OClO, is examined under isolated molecule conditions and in water solution. Reaction in the ground state leads to Cl( 2P u) + O 2( 3Σg −). Within experimental error, the barrier to reaction in water solution is the same as that estimated for the reaction in gas phase. Chemistry on the excited electronic manifold occurs via multiple pathways leading to ClO( 2Π) + O( 3P g), Cl( 2P u) + O 2( 3Σ g −) and Cl( 2P u) + O 2( 1Δ g). The excited state reactivity of OClO in water solution is significantly different from the gas phase process. The effect of solvent on the excited state photoreactivity of OClO is discussed in detail.

Activated barrier crossing: Comparison of experiment and theory

Photochemical isomerization in stilbene and diphenyl butadiene has been studied as a model for activated barrier crossing. Experiments have been carried out from isolated molecule conditions up to 3000 atm pressure in solution-phase samples. The qualitative features predicted by Kramers theory are observed. The system undergoes a transition from energy-controlled to diffusion-controlled behavior in the high-pressure gas phase. The influences of multidimensionality, intramolecular vibrational relaxation, and frequency dependent friction are discussed.

Picosecond‐Absorption Study of the Photoisomerization of Trans‐Stilbene in Compressed Gases and Liquids

AbstractThe photoisomerization of trans‐stilbene after picosecond excitation at 308 nm has been studied by picosecond absorption at 500 or 616 nm in ethane and propane. Experiments were performed in gaseous ethane (30–54 bar), liquid ethane (42–6000 bar), and liquid propane (9–4050 bar). The pressure dependence of the reaction is modelled from isolated molecule conditions in low pressure gases up to diffusion control in high pressure liquids. In dense media marked solvent shifts of the reaction threshold energy are postulated which can be described by simple cluster — and cage — solvent shift models. UV spectra show similar solvent shifts as isomerization rates.

ChemInform Abstract: PHOTOPHYSICS OF LASER‐EXCITED THIOFORMALDEHYDE A(1A2)

Fluorescence has been excited in the rotational subband heads of low lying vibrational levels in the A(1A2) state of thioformaldehyde. Single exponential fluorescence decays measured as a function of pressure yield linear Stern–Volmer plots from which gas kinetic quenching rates and collision‐free lifetimes of between 14 (000 band) and 4 μs (420 band) have been obtained. The results are interpreted in terms of solely radiative decay of the vibronic states investigated under isolated molecule conditions.

Photophysics of laser-excited thioformaldehyde Ã(1A2)

Fluorescence has been excited in the rotational subband heads of low lying vibrational levels in the Ã(1A2) state of thioformaldehyde. Single exponential fluorescence decays measured as a function of pressure yield linear Stern–Volmer plots from which gas kinetic quenching rates and collision-free lifetimes of between 14 (000 band) and 4 μs (420 band) have been obtained. The results are interpreted in terms of solely radiative decay of the vibronic states investigated under isolated molecule conditions.

Position dependent deuterium effects on the vapor phase fluorescence lifetimes of tetradeuterophenanthrenes

The fluorescence lifetime of phenanthrene and three of its tetradeuterated isomers are measured in the vapor phase under isolated molecule conditions. The lifetimes fall in the order phenanthrene < 1,3,6,8- d 4 < 1,2,3,4- d 4 < 2,4,5, 7- d 4. The possibility that the change in lifetimes is due to a resonance mechanism of intersystem crossing is discussed.

Second excited singlet state fluorescence of CIFCS

Absorption, fluorescence emission and fluorescence excitation spectra of the B 1A'← X 1A'(π * ← π) band system of CIFCS in the gas phase at room temperature and at −78°C have been measured. Quantum yields of fluorescence from nine single vibronic levels of the B state have been measured under isolated molecule conditions and radiative and non-radiative decay constants have been estimated.

The electronic relaxation of benzophenone in the vapor phase

The time-resolved luminescence of benzophenone was studied for two excitation wavelengths as a function of pressure. In isolated molecule conditions a single microsecond emission was observed. At increasing pressure the lifetime of this emission becomes shorter, while simultaneously a millisecond emission shows up, its intensity increasing with pressure. The microsecond emission was identified as a combination of hot phosphorescence and diluted fluorescence of the singlet—triplet scrambled molecular eigenstates, the millisecond emission as the normal thermalized phophorescence. From the analysis that is possible after this identification it appears that both radiative and nonradiative rates increase with excitation energy. The rate of formation of thermalized triplet levels is about 20% of the molecular collision rate.

Position dependent deuterium effects on the vapor phase fluorescence lifetimes of dideutero phenanthrenes

The fluorescence lifetimes of phenanthrene and three of its dideutero isomers are measured in the vapor phase under isolated molecule conditions with a bandpass of 25 cm 1. The lifetimes fall in the order phenanthrene > 2,4- d 2 > 1,3- d 2 > 9,10- d 2. It is suggested that the change in lifetimes is due to a resonance mechanism of intersystem crossing.

Delocalization of electronic energy in the lowest triplet states of molecules

A variety of molecules are examined for the existence of metastable (τ≳1 msec) electronic states under isolated molecule conditions. A beam is excited by electron impact and detected after a 1 msec drift by Auger electron emission from a cesium surface. Metastable states were observed for benzene, toluene, m-xylene, acteylene, cyanogen, SO2, N2, and CO. Molecules observed to not have metastable electronically excited states were formaldehyde, acetone, cyclohexanone, 2-cyclopenteneone, 2-cyclohexeneone, bromobenzene, benzylbromide, propyne, hydrogen cyanide, acetonitrile, carbon dioxide, carbon disulfide, carbonyl sulfide, ammonia, ethylene, 1,3-butadiene, cis-2-butene, trans-2-butene, cyclopentene, cyclohexene, cyclopropane, and allene. A correlation is observed between nonmetastability of the lowest triplet state and a large change in geometry between the ground electronic state and the lowest triplet state. Mixing of the lowest triplet state of a molecule with high vibrational levels of the ground electronic state is discussed in light of the experimental findings. The generalized Franck–Condon factors of the presently accepted theory of radiationless transitons are found to be the crucial factors which determine the metastability or nonmetastability of the lowest triplet state of most molecules. Cross sections for production of high vibrational levels of the ground electronic state (Evib≳20 000 cm−1) by electron bombardment excitation of a triplet state which is strongly coupled to these levels of the ground state may be as large as a few tenths of a square angstrom.

Two-photon absorption in the collisionless gas phase: lifetimes of new vibrational levels in benzene

Two-photon photoselection has been demonstrated for a molecule in the low pressure gas phase. The excitation has been used to populate new vibrational levels in benzene under isolated molecule conditions down to 0.1 torr. The new states display unusual behaviour as compared to the lifetimes of one-photon excited states. Most unexpectedly the lifetimes of the strongest two-photon inducing mode ν 14 is longer than all other lifetimes, including that of the vibrationless 1B 2u origin. Some theoretical explanations are offered.