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The conformational structures of heterocyclic compounds are of considerable interest to chemists and biochemists as they are often the constituents of natural products. Among saturated four-membered heterocycles, the conformational structure of oxetane is known to be slightly puckered in equilibrium because of a low interconversion barrier in its ring-puckering potential, unlike cyclobutane and thietane. We measured the one-photon vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) and two-photon IR+VUV-MATI spectra of oxetane for the first time to determine the ring-puckering potential of the oxetane cation and hence its conformational structure in the D0 (ground) state. Remarkably, negative anharmonicity and large amplitudes were observed for the ring-puckering vibrational mode progression in the low-frequency region of the observed MATI spectra. We were able to successfully analyze the progression in the MATI spectra through the Franck–Condon simulations, using modeled potential energy functions for the ring-puckering modes in the S0 and D0 states. Considering that the interconversion barrier and puckered angle for the ring-puckering potential on the S0 state were found to be 15.5 cm−1 and 14°, respectively, the cationic structure is expected to be planar with C2v symmetry. Our results revealed that the removal of an electron from the nonbonding orbitals on the oxygen atom in oxetane induced the straightening of the puckered ring in the cation owing to an increase in ring strain. Consequently, we conclude that this change in the conformational structure upon ionization generated the ring-puckering vibrational mode progression in the MATI spectra. [ABSTRACT FROM AUTHOR]

We report conformational and vibrational assignments of vacuum ultraviolet mass-analyzed threshold ionization spectrum of the isolated gauche conformer, based on previously determined conformer-specific photoionization and conformational stabilities of isobutanal. The vibrational spectrum of the pure cationic gauche conformer was acquired by removing the trans conformer via conformationally effective cooling with Ar carrier gas. The peaks in the spectrum were assigned by Franck-Condon (FC) fitting by adjusting the cationic geometrical parameters of the gauche conformer at the CAM-B3LYP/cc-pVTZ level. Based on the good agreement between the experimental and calculated results, we were able to determine the precise structure of the cationic gauche conformer of isobutanal with C1 symmetry. Notably, the unveiled vibrational structure was mainly attributed to a geometrical change along the vibrational motions associated with the formyl torsion and CC stretching upon ionization, resulting in their prominent spectral overtones and combination bands with other fundamental vibrations. On the potential energy curve for the formyl torsion of the cationic gauche conformer determined by FC fitting, the transition barrier at 313 cm−1 preserved the hindered formyl torsion in the case of a harmonic potential well, which was confirmed by the progression of formyl torsion, namely, 331, 332, and 333 observed at 60, 120, and 180 cm−1, respectively. [ABSTRACT FROM AUTHOR]

Isobutanal is an aliphatic aldehyde which has been extensively studied as an important intermediate in isomerization reactions as well as in astrochemically relevant models in the interstellar medium. Herein, we report on the conformer-specific photoionization and conformational stabilities of isobutanal utilizing one-photon mass-analyzed threshold ionization (MATI) spectroscopy with vacuum ultraviolet (VUV) pulses. The conformational population of isobutanal under different supersonic expansion conditions was explored to identify the conformers, from which their intrinsic photoionizations can be directly elucidated by measuring the VUV-MATI spectrum corresponding to each conformer. The observed MATI spectra could be analyzed through Franck–Condon simulations at the B3LYP/cc-pVTZ level for the isobutanal conformers, trans and gauche, for which the adiabatic ionization energies were precisely determined to be 78 133 ± 3 cm−1 (9.6873 ± 0.0004 eV) and 78 557 ± 3 cm−1 (9.7398 ± 0.0004 eV), respectively. Notably, only the gauche conformer undergoes a unique geometrical change upon ionization, resulting in the progression of the CHO torsional mode in the MATI spectra. Consequently, we determined the conformational stabilities of isobutanal by conformer-specific photoionization, given that the gauche is more stable than the trans by 162 ± 50 cm−1 in the neutral ground state, while the cationic gauche is less stable than the cationic trans by 262 ± 50 cm−1. [ABSTRACT FROM AUTHOR]

Vibrational spectra of C[SUB6]H[SUP+,SUB6] and C[SUB6]D[SUP+,SUB6] in the ground electronic state have been measured by one-photon mass-analyzed threshold ionization (MATI) spectroscopy using coherent vacuum ultraviolet radiation generated by four wave mixing in Kr gas. The ionization energies of C[SUB6]H[SUB6] and C[SUB6]D[SAB6] determined by one-photon MATI, 74551 ± 5 and 74579 ± 5 cm[SUP-1], respectively, are similar to those reported previously. Vibrational spectra are much simpler than the previous zero kinetic energy photoelectron and MATI spectra obtained by two-photon excitation. Almost complete vibrational assignments for the cations have been possible, which will be useful for future theoretical studies of the Jahn-Teller effect in these cations. Implication from the present one-photon spectra agrees with the previous suggestion that the geometry of benzene cation in the ground electronic state belongs to the D[SUB6h] symmetry. [ABSTRACT FROM AUTHOR]

Vibrational spectra of p-, m-, and o-difluorobenzene cations in the ground electronic states have been measured by vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. From the MATI spectra, ionization energies of p-, m-, and o-difluorobenzenes to the ionic ground states have been determined to be 73861 ± 5, 75332 ± 5, and 75003 ± 5 cm[SUP-1], respectively. A number of vibrational modes, including fundamentals, their combinations, and overtones were distinctly observed in the MATI spectra and could be assigned successfully by comparing with the results from density functional theory calculation at the B3LYP/6-311 + + G (2df ,2pd) level. Appearance of the overtone progression of some totally symmetric vibrations could be explained also in terms of the overlap between the geometrical changes upon ionization and normal mode eigenvectors. Excellent correlation between the experimental and calculated results suggests that p-, m-, and o-difluorobenzene cations in the ground states retain D[SUB2h], C[SUB2v], and C[SUB2v] symmetries, respectively, of the corresponding neutrals. [ABSTRACT FROM AUTHOR]

Vibrational spectra in the ground electronic states, &Xtilde;²B[sub 1], of fluoro-, chloro-, bromo-, and iodobenzene cations and in the excited electronic states, &Btilde;²B[sub 2], of chloro- and bromobenzene cations have been measured by mass-analyzed threshold ionization (MATT) via one-photon excitation with vacuum ultraviolet light. The spectra consist mostly of strong 0-0 bands and fundamentals, overtones, and combinations of totally symmetric a l modes. In particular, the observed vibrational bands in the B states have been assigned by referring to the assignments for the ionic ground states and those for the neutral molecules. From the MATT spectra, the ionization energies of fluoro-, chloro-, bromo-, and iodobenzene to the ionic ground states have been determined to be 74229±5, 73177±5, 72570±5, and 70638±+5 cm[sup -1], respectively. The ionization energies to the B states have also been determined to be 91404±5 and 85 822 ± 5 cm [sup -1] for chloro- and bromobenzene, respectively. [ABSTRACT FROM AUTHOR]

Final state resolved vector properties of the reaction H (2S)+O2(3Σ-g)→OH(2Π)+O(3P) have been measured using Doppler spectroscopy. Velocity-aligned H atoms were generated by photodissociation of H2S with linearly polarized ArF excimer laser light. The correlated velocity and angular momentum distribution of nascent OH reaction products was probed by laser induced fluorescence. Techniques are described for analyzing the line shapes, modeling the bipolar moments of the OH velocity and angular momentum distribution as parametric functions of the scattering angle. Distinctive Doppler-broadened lineshapes indicate scattering angle distributions with a strong forward and a weaker backward maximum. A qualitatively different angular momentum polarization for the two lambda doublet components of the OH product is required to explain the Doppler profiles. [ABSTRACT FROM AUTHOR]

The effect of rotational excitation on intramolecular vibrational relaxation in isolated gas phase molecules of dimethyl ether and 1,4 dioxane has been studied. Frequency resolved infrared fluorescence spectra were obtained from molecules isolated in a supersonic expansion and excited in the CH stretching region with an infrared optical parametric oscillator. By altering the expansion conditions the average J of the molecules was controlled. Measurements performed on both molecules at two different rotational temperatures indicate that the dilution factors of the CH stretch modes have a significant J dependence. It is demonstrated that the measured energy contents of the relaxed modes can be satisfactorily modeled by a statistical calculation which assumes equal coupling of all modes with 1 cm-1 of the optical state which conserve J and symmetry. It is shown that the ultimate distribution of the energy within the molecule is J dependent. [ABSTRACT FROM AUTHOR]

The phenomenon of interference of the amplitude for absorption of one photon of frequency 3ω and the amplitude for absorption of three photons of frequency ω was theoretically predicted by Shapiro, Hepburn, and Brumer. The interference was demonstrated experimentally by varying the relative phase between the tripled frequency photon and three photons with the fundamental frequency by the groups of Elliott and Gordon in atoms and small molecules. In order to see how general this phenomenon is, five compounds were studied, ammonia, trimethylamine, triethylamine, cyclooctatetraene, and 1,1-dimethylhydrazine. CH3I was used as the tripling gas for light in the range 604–600 nm. Interference was observed in all cases. The last four compounds have low ionization potentials and interference was observed between a 3+1 and a 1+1 ionization process with a maximum modulation of 22%. NH3 with a higher ionization potential requires absorption of 3+2 or 1+2 photons and exhibits a maximum modulation of 33%. We conclude that molecular size is no obstacle and that as long as a molecule has sufficiently strong absorption at the tripled frequency, and sufficient vapor pressure, and the laser fundamental beam is very strong, phase control of interference is observable. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photodissociation of iodine monochloride has been investigated at 235–248 nm by studying Doppler profiles of (2+1) resonance enhanced multiphoton ionization spectra of fragment chlorine atoms and two-photon laser induced fluorescence spectra of iodine atoms. At 235.3–237.8 nm, a measured branching ratio of [Cl*(2P1/2)]/[Cl(2P3/2)] is 0.68±0.10. At 248 nm, the [I*(2P1/2)]/[I(2P3/2)] ratio is 0.71±0.27. From the measured Doppler profiles and the ab initio calculation, the photoprepared states leading to Cl and Cl* production are assigned to the 3Π0+ (and possibly 3∑-0+) and the 1Π1 states, respectively. Nonadiabatic couplings between the potential curves have been discussed. [ABSTRACT FROM AUTHOR]

O(2p3Pj) atoms produced from the reaction between H(1s2S) and O2(X3Σ-g) at collision energies (Ecoll) of 2.5 and 2.1 eV are detected by a resonance enhanced multiphoton ionization technique. From the Doppler profile measurements of O(3Pj), the average kinetic energy released to the products is estimated. The results suggest that the average fraction of the kinetic energy disposal at Ecoll =2.1 eV is larger than that at Ecoll =2.5 eV. The obtained branching ratios of O(3Pj) j=2, 1, and 0 at Ecoll =2.5 eV are (0.70±0.02), (0.21±0.02), and (0.09±0.02), and those at Ecoll =2.1 eV are (0.63±0.02), (0.25±0.04), and (0.12±0.02), respectively, with error bars of ±σ. The results suggest that the nonadiabatic couplings at the last stage of the reaction are important for the determination of the ratios. [ABSTRACT FROM AUTHOR]

State-selected photodissociation dynamics of CS[sub 2][sup +] molecular ion has been investigated by an optical-optical double resonance technique. The CS[sub 2][sup +] molecular ions were prepared by [2+1] REMPI and the predissociative C˜ [sup 2]Σ[sub g][sup +] state was populated by the B˜ [sup 2]Σ[sub u][sup +]←X˜ [sup 2]Π[sub g,3/2] followed by C˜ [sup 2]Σ[sub g][sup +]←B˜ [sup 2]Σ[sub u][sup +] transition. The product branching ratios, CS[sup +]/S[sup +], for selected vibrational levels in the C˜ state and average kinetic energy releases in the CS[sup +] and S[sup +] production channels were measured from the time-of-flight mass spectra. It has been found that excitation of the bending vibration enhances the CS[sup +] production channel more than the S[sup +] channel. In addition, an isotropic fragment distribution for different polarizations of the dissociating laser light has been observed, from which the dissociation time was estimated to be longer than 20 ps. Dynamical aspect of the reaction revealed by the present investigation is discussed together with the previous spectroscopic results. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ionization energies and cationic structures of pyridine were intensively investigated utilizing one-photon mass-analyzed threshold ionization (MATI) spectroscopy with vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. The present one-photon high-resolution MATI spectrum of pyridine demonstrated a much finer and richer vibrational structure than that of the previously reported two-photon MATI spectrum. From the MATI spectrum and photoionization efficiency curve, the accurate ionization energy of the ionic ground state of pyridine was confidently determined to be 73,570 ± 6 cm(-1) (9.1215 ± 0.0007 eV). The observed spectrum was almost completely assigned by utilizing Franck-Condon factors and vibrational frequencies calculated through adjustments of the geometrical parameters of cationic pyridine at the B3LYP/cc-pVTZ level. A unique feature unveiled through rigorous analysis was the prominent progression of the 10 vibrational mode, which corresponds to in-plane ring bending, and the combination of other totally symmetric fundamentals with the ring bending overtones, which contribute to the geometrical change upon ionization. Notably, the remaining peaks originate from the upper electronic state ((2)A2), as predicted by high-resolution photoelectron spectroscopy studies and symmetry-adapted cluster configuration interaction calculations. Based on the quantitatively good agreement between the experimental and calculated results, it was concluded that upon ionization the pyridine cation in the ground electronic state should have a planar structure of C(2v) symmetry through the C-N axis.