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Fourier transform infrared (FTIR) absorption spectra in the 2000 to 3500 cm–1 range are reported for the gaseous 16 O, 16 O- and 18 O, 18 O-isotopomers of tropolone[OH(OD)] at 25 oC. The spectral doublet component separations are near 20 and 19 cm–1 for 16 O, 16 O- and 18 O, 18 O-Tp(OH), respectively, and near 7 and 6.5 cm–1 for 16 O, 16 O- and 18 O, 18 O-Tp(OD). The spectra suggest the tautomerization tunneling mechanisms in these states are complex multidimensional processes including the participation of IVR. In general, the OHO isotope effects demonstrate a mixing of O atom displacement coordinates into the intramolecular dynamics for most of the vibrational states observed in the fundamental CH/OH(OD) stretching regions.

Computational studies of 12 64-electron homologues and isomers of tropolone in the S(0) electronic ground state are reported. Three minimum-energy structures, tropolone (Tp), 5-azatropolone (5Azt), and 5-H-5-azatropolonium (5AztH(+)), have an internal H-bond and planar C(s)) geometry, and three, tropolonate (TpO(-)), 5-azatropolonate (5AzO(-)), and 1-H-azepine-4,5-dione (45Di), lack the H-bond and have twisted C(2) geometry. All 6 substances have an equal double-minimum potential energy surface and a saddle point with planar C(2)(v) geometry. The energy for the gas-phase isomerization reaction 45Di --5Azt is near +4 kJ mol(-1) at the MP4(SDQ)/6-311++G(df,pd)//MP2/6-311++G(df,pd) (energy//geometry) theoretical level and around -20 kJ mol(-1) at lower theoretical levels. The dipole moments computed for 45Di and 5Azt are 9.6 and 2.1 D, respectively, and this large difference contributes to MO-computed free energies of solvation that strongly favor--as experimentally observed--45Di over 5Azt in chloroform solvent. The MO-computed energy for the gas-phase protonation reaction 45Di + H(+) --5AztH(+) is -956.4 kJ mol(-1), leading to 926.8 kJ mol(-1) as the estimated proton affinity for 45Di at 298 K and 1 atm. The intramolecular dynamical properties predicted for 5Azt and 5AztH(+) parallel those observed for tropolone. They are therefore expected to exhibit spectral tunneling doublets. Once they are synthesized, they should contribute importantly to the understanding of multidimensional intramolecular H transfer and dynamical coupling processes.

Theoretical studies have been carried out to elucidate the ground-state unimolecular decomposition channels for propynoic acid (PA) in the gas phase. In this paper we present details of mechanisms for the decarboxylation and decarbonylation channels of PA. The transformations of PA result in the formation of carbon dioxide and acetylene from the decarboxylation channel while carbon monoxide and hydroxyacetylene, ultimately the ketene, are formed from the decarbonylation pathway. Equilibrium structures and the principal transition states (TSs) for each pathway have been identified and characterized. The Gaussian 98 suite of programs and MOLDEN were used for computation and visualization. All computations of equilibrium and TS structures relevant to the two competing unimolecular decomposition channels (decarboxylation and decarbonylation) were performed with second-order Moller–Plesset (MP2) and Becke's three-parameter exchange functional and the gradient-corrected functional of Lee, Yang, and Paar (B3LYP) levels using the 6-31G(d,p), 6-311++G (d, p) and Dunning's correlation-consistent polarized valence basis set (aug-cc-pVDZ). The geometries were fully optimized and characterized as minima (0 imaginary frequencies) or first-order saddle points (1 imaginary frequency) by harmonic vibrational analysis at the MP2/6-311++G(d,p) and B3LYP/6-311++G(d,p) levels. All the calculations indicate that the lowest energy decomposition pathway for PA is decarboxylation. Decarboxylation of PA occurs most easily through a three-step mechanism: conversion of s-cis-HCCCO2H to s-trans-HCCCO2H, followed by a 1, 3-hydrogen migration of hydroxyl hydrogen, resulting in a four-center TS that decays easily to acetylene and carbon dioxide, with an activation barrier of 61.3–79.9 kcal/mol (average 65.67 kcal/mol). Decarbonylation, leading to hydroxyacetylene (and subsequently the ketene) and carbon monoxide, occurs easily via a direct (one-step) three-center TS. The direct (one-step) elimination of carbon monoxide from propynoic acid has a calculated average activation barrier of 67.9–81.7 kcal/mol (average, 74.1 kcal/mol). The decarboxylation pathway is shown to occur with a maximum exothermicity of 28 kcal/mol. On the contrary, the decarbonylation of PA is predicted to occur with an overall endothermicity of ≈29.2 kcal/mol for formation of hydroxyacetylene and carbon monoxide and an average exothermicity of −7.69 kcal/mol for formation of the ketene and carbon monoxide. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Nitrogen pressure broadening effects are observed on the high resolution ( 0.0025 cm −1 ) FTIR spectrum of gaseous tropolone at 298 K and 32 m of path length. Broadening observations on narrow absorption spikes contribute to understanding of the spectroscopy and dynamics of tropolone in several ways. First, they help to establish a remarkable progression of sharp peaks near 752 cm −1 as a part of the Q branch of the anharmonic ν22 (COH torsion) fundamental. The subband spacings of about 0.005–0.010 cm −1 are too large to be determined by harmonic vibrational differences in the rotational constants alone and are attributed to vibration–contortion–rotation perturbations of the upper state (ν22) energy levels of this anharmonic and relatively large amplitude vibration. Second, the isolated and sharp Q branch spikes near 915 cm −1 allow first estimates to be made for effective composite pressure broadening coefficients of tropolone–N2 collisions. The coefficients for transitions within the Ncon=0 and Ncon=1 contortion states differ from each other. They are relatively small due to the preponderance of high rotational states in their composition. Third, N2 broadening effects on substructure in the Q branch doublet near 754 cm −1 help support the assignment of this doublet to tunneling structure of the ν37 fundamental (contortion or incipient skeletal tunneling). The observed doublet components appear as perturbatively IR enhanced three and five peak fragments of weak longer progressions. The parallel spectroscopic behaviors of the perturbed anharmonic ν22 and ν37 vibrations differ markedly from behavior observed for the quasiharmonic vibrations.

The infrared absorption spectrum of tropolone vapor at 25 °C, ∼0.01 Torr, and 32 m path length has been recorded from 960 to about 700 cm-1 at a resolution of 0.0025 cm-1. Twenty-nine cold band and hot band spectral tunneling doublets marked by sharp type A or type C Q branch apexes protruding from the congested vibration−contortion−rotation absorption profiles are assigned. Twenty-six vibration-contortion state-specific splittings are estimated for tropolone in its ground electronic state. This number is currently unprecedented in the literature for any sizable molecule. About half of these quasiharmonic vibrational states show an appreciable quenching of tunneling due to increased effective barriers and/or path lengths and, in the case of the nominal COH torsion, the tunneling is reduced to 0.07 of the zero-point (ZP) value of 0.974 cm-1. The analysis is guided by predictions of the independent {tunneling skeleton}{tunneling H atom} tautomerization model that was previously applied to the vibrational sp...

The minimum energy pathway leading between the tautomers of tropolone was calculated using molecular orbital (MO) methods. This, with various 1D and 2D cuts of the potential energy surface (PES) topography, reveals the {tunneling skeleton}/{tunneling H atom} mechanism for tautomerization. In the zero-point states the H atom is localized to one of the O atoms until the tropolone skeleton becomes sufficiently vibrationally displaced towards C2v configurations that near-equal double-minimum potential energy functions (PEFs) arise for the H atom vibration. The resulting delocalization of the H atom between the two O atom sites allows the skeletal displacement to proceed through the barrier and the tautomerization process to be completed. The v1 (OH stretching) energies in quantum states N1 are strongly dependent on the skeletal geometry and, adiabatically separated from the slow v22 vibration, they contribute to markedly different 1D effective potential energy functions V22eff[N1] for v22. V22eff[N1=0] is a n...

Infrared spectra of tropolone(OH) and tropolone(OD) obtained from vapor phase, solvated, and rare gas matrix-isolated samples, and from fluorescence dip infrared spectroscopy experiments by Frost et al. on jet-cooled samples, are analyzed with the guidance of high level ab initio molecular orbital (MO) computations. It is found that the anharmonicity of the double minimum global potential energy surface of S0 tropolone is manifested by multistate local resonance networks coupling fundamental vibrations to nearby overtone and combination states. These resonance networks pervade the IR spectrum of tropolone above 500 cm−1, and the absorbances are much more strongly perturbed from harmonic level predictions than the frequencies. Some of the IR absorbances are also sensitive to intermolecular interactions. At maximum spectral resolutions reaching ∼0.2 cm−1 only the v1 and v22 (OH stretching and nascent skeletal tunneling) vibrations show resolved vibrational state-specific tunneling doublets. The tunneling be...

The symmetrical geometries of 1,3-cyclobutadiene (CB) and substitutional vacancies in crystalline fcc Ar allow purely ab initio MO estimations to be made for the stabilization energies of several symmetric trapping sites in the Ar/CB matrix system at the MP2/6-311++G(2d,2p) [denoted HI-MO] level of theory. The single-substitutional vacancy supports the most stable trapping site in the HI-MO Ar/CB system. It was examined by filling an Ar13 vacancy of the HI-MO Ar lattice with a symmetrical CB:Ar12 kernel cluster, with dilation of the kernel cluster opposed by a symmetrical HI-MO lattice distortion PEF derived for the purpose. In addition, various supplemental HI-MO stabilization energies were computed. Slightly displacing the four nearest-neighbor Ar atoms coplanar with CB from square-planar geometry shows this site distortion PEF is very soft. The results suggest fluxional behavior for the Ar atoms near the CB guest molecule and the soft trapping site environment provides access to equivalent global PES m...

MO computations of vibrational spectra for 1,3-cyclobutadiene (CB) and CB:CO2 van der Waals molecule isotopomers at the MP2/6-311++G(2d,2p) level of MO theory are analyzed together with the extensive Ar/CB matrix-isolation spectra of the Michl group, the low 3.2 kcal/mol (ZPE-corrected) MO-computed energy of the square-planar saddle-point for CB reported by Nakamura et al., and opportune one-dimensional analysis of the automerization energy levels of 12C4H4. The composite interpretation results in the assignment of the strongly anharmonic Raman transitions of Ar/CB observed at 723 and 1678 cm−1 to the E2-E0 and E4-E0 transitions, respectively, in the automerization coordinate. The 13C and D isotope dependencies of the nominal C=C stretching and δCH wagging vibrational normal modes of the CB valence isomers depend markedly on whether the labeled HC=CH moieties of CB are mass equivalent or not, and the pronounced normal mode differences are viewed as providing different gateways into the global anharmonic PES domains. The automerization coordinate Qa originates as the nominal C=C stretching normal coordinate of ag symmetry, followed by a curving to more efficiently interchange the C=C and C–C bond lengths as the square-planar D4h configuration is approached. The latter is crossed at a geometry dilated with respect to the saddle-point, thereby providing a barrier configuration about 5 kcal/mol above the PES minima (i.e., a dilation energy of about 2 kcal/mol plus the 3.2 kcal/mol saddle-point energy). The computed zero-point levels of 12C4H4 (1-D modeling) are separated by E1-E0=11 cm−1 to suggest ZP tunneling rates reaching the picosecond time scale. The analysis exposes new aspects of the intramolecular dynamics of CB, and the proposed automerization mechanism accounts for all of the presently known information pertaining to this interesting property of the cyclobutadiene molecule.

Fourier transform infrared absorption spectra containing evidence for about two dozen spectral tunneling doublets are reported for gaseous tropolone(OH), tropolone (OD), and 18O,18O-tropolone(OH) in the 800 to 300 cm-1 spectral range. No FTIR absorption was detected in the 300-150 cm-1 range. The known zero-point (ZP) tunneling splitting values Delta0 = 0.974 cm-1 for tropolone(OH) (Tanaka et al.) and 0.051 cm-1 for tropolone(OD) (Keske et al.) allow vibrational state-specific tunneling splittings Deltav to be estimated for fundamentals including three with strong O...O stretching displacements [cf. for tropolone(OH) nu13(a1) = 435.22 cm-1 with HDelta13 = 1.71 cm-1 = 1.76 HDelta0, and for tropolone(OD) nu13(a1) = 429.65 cm-1 with DDelta13 = 0.32 cm-1 = 6.27 DDelta0]. The majority of Deltav splittings in the sub-800 cm-1 range are dilated relative to the isotopomer Delta0 values. The FTIR spectra demonstrate the presence of dynamic couplings and potential function anharmonicity in addition to revealing Deltav splittings and many OH/D and 18O/16O isotope effects. Approximate values are obtained for the ZP splittings 88Delta0 and 86Delta0 of the doubly and singly 18O-labeled isotopomers of tropolone(OH). The diverse values of the observed Deltav/Delta0 splitting ratios underscore the inherent multidimensionality and corner-cutting activities entering the state-specific tunneling processes of the tropolone tautomerization reaction.

Properties of dihydroxycarbene in its S 0 ( 1 A 1 ) and T 1 ( 3 B ) electronic states are investigated using MO methods at basis-set levels including polarization functions and approximations for electron correlation. The energetics of addition reactions and the properties of adducts formed between (HO) 2 C and H 2 , NO, N 2 , O 2 or H 2 O are considered. The adducts include methanediol and methanetriol, 3,3-dihydroxydiazirine and dihydroxydiazomethane, dihydroxynitrosomethyl, singlet dihydroxyoxirane, triplet dihydroxydioxymethane, and the dihydroxyketone oxide (HO) 2 COO.

Laser excitation spectra with v=0, 2, 4, and 6 in the A 1B2–X 1A1 26v0 progression of jet‐cooled 18O/16O isotopomers of tropolone are reported. The isotope shift for ν26, an out‐of‐plane deformation mode at 39 cm−1 in the A state, is 2% for tropolone‐18O18O. This large 18O isotope effect indicates that Q26 for tropolone resembles the analogous normal mode of tropone, which is a ring deformation towards the boat conformation of 2, 4, 6‐cycloheptatriene accompanied by a large O atom displacement. Tunneling by tropolone in the A state is quenched by exciting the 26v overtone states and a mechanism for this quenching is proposed in terms of the indicated normal coordinate. Tunneling splittings are

Calculs optimises ab initio MO(6-31G* de la geometrie et du moment dipolaire. Calculs 6-31G du spectre vibrationnel, des intensites IR et Raman

Infrared-absorption profiles observed for vibrational transitions of gaseous tropolone often show sharp Q branch peaks, some of them ultranarrow spikes, indicative of the band origins for vibrational state-specific spectral tunneling doublets. In this work oxygen isotope effects for two CH wagging fundamentals, the COH torsion fundamental, and the skeletal contortion fundamental are reported. They allow considerations to be given: (1) oxygen isotope effects on the vibrational frequencies and state-specific tunneling splittings; (2) the asymmetry offset of the potential-energy minima for O16 and O18 tropolone; and (3) additional details concerning previously proposed high J rotation-contortion resonances in the contortional fundamental. The new results help to characterize the skeletal contortion fundamental and support the joint participation of skeletal tunneling with H tunneling in the vibrational state-specific tautomerization processes of tropolone in its ground electronic state.

Large (6.9 to 16.3 cm−1 ) tunneling splittings are uniquely observed for the ν27 (OD stretch), ν31 (carbonyl stretch), and ν34 (C=C–C stretch) fundamentals of tropolone‐OH and tropolone‐OD in the X 1A1 (ground) electronic state. These same three modes are predicted by the molecular geometry to interact strongly with tunneling because the dominant vibrational and tunneling displacements involve the same atoms. The heavy atom tunneling displacements (≊0.07 A) are small enough to plausibly consider heavy atom tunneling phenomena—especially in appropriate excited vibrational states—and the tunneling splittings appear consistent with behavior expected at zero order for adiabatic reaction surface theory with a 2D reaction surface defined by C=O/C–O and C=C–C heavy atom coordinates. This model attributes tunneling in the X state of tropolone to heavy atom motion followed adiabatically by H atom motion rather than the reverse. Energy balance equations are used to obtain estimates for the vibrational state‐speci...

Infrared spectra of glyoxylic acid monomers, including the hydroxyl deuterated and general O-18 isotopomers, are reported for argon and nitrogen matrix-isolated samples in the range 400–4000 cm−1. Values for all 15 fundamental vibrations of the intramolecular H-bonded monomer are listed. Bands attributed to the trans-carbonyl conformer with a free car☐yl group are also observed and about half of its fundamental frequencies are listed. Some modes of the intramolecular H-bonded glyoxylic acid monomer are strongly matrix dependent, and differences between Ar and N2 matrix spectra are exploited in sorting out the spectral information.

Infrared spectra of monomeric oxalic acid-h2, -hd and -d2 have been observed using neon matrix-isolation spectroscopy. In addition, spectra of oxalic acid-h2, vapor were obtained using a heated absorption cell with a 10 m pathlength. All IR active fundamentals are assigned for oxalic acid-h2, and -d2, except the low frequency torsion. The spectra are interpreted in terms of the C2h intramolecularly hydrogen bonded model. Two vibrational assignments are discussed; one involves a very large intensity for the v8 + v11 (COH torsion) combination band of oxalic acid-h2. Tentative values for several Raman active fundamentals of oxalic acid-h2 are suggested using possible combination bands. Few fundamentals of oxalic acid-hd correlating with the Raman modes of the symmetric monomers could be observed. A force constant analysis of the by secular block is presented.

A previously undetected perturbation in the HCCH A 1Au 3ν3 (trans‐bend) level has been found and analyzed. The perturbing level is identified as the A 1Au ν2+2ν4 level. Locally deperturbed molecular constants for the 3ν3 and perturbing level are obtained, along with the perturbation matrix element and the J‐dependent mixing coefficients. The A‐state torsional fundamental, ν4≈850 cm−1, is estimated from the energy of perturbing level. The presence of the perturbation explains large differences between the rotational line intensity patterns observed in the absorption and fluorescence excitation spectra. The perturbed 3ν3 and extra lines are especially prominent in a fluorescence excitation spectrum of the A 1Au–X 1Σ+g 33 0 K10 subband of acetylene rotationally cooled in a supersonic jet.

Present address of Duane R, Kirklin: National Bureau of Standards, Washington, D. C., 20234. Ellis R, Lippincott: Posthumous.

Six recently reported intermolecular potential energy functions for HF–HF interactions are examined and used to calculate classical second virial coefficient curves. Order of magnitude differences exist among the six B(T) curves and none of them decreases with temperature as rapidly as an experimental curve determined in the 19.5–56 °C range. The HF3 potential of Klein, McDonald, and O’Shea appears to give the best overall description of HF–HF interactions and this function is used to calculate properties of several higher (HF)q oligomers. The resulting low energy conformations are cyclic, and the hexamer has a puckered, nonrigid structure. The calculated hydrogen bond energies, F⋅⋅⋅F distances, and the puckered, nonrigid hexamer structure all agree well with the available experimental information.

Five of the lowest frequency species a1, b1, and b2 vibrational modes of tropolone in the A 1B2 electronic state are studied using high resolution laser fluorescence excitation spectroscopy of the jet‐cooled sample. The ν’26(b1) mode at 38 cm−1, most probably a ‘‘folding’’ vibration of the seven‐membered and O⋅⋅⋅HO chelated ring systems of tropolone, is observed in the progression 26v0 with primarily even valued quantum numbers to v26=14. Tunneling doublets in the progression are observed to decrease from 18.90 cm−1 in the vibrationless state to unresolvable values when v’26 is larger than 8, thereby demonstrating highly mode‐specific quenching of the ‘‘H atom’’ tunneling process by these low frequency, out‐of‐plane, ring deformation vibrations. The progression 251026v0 is observed to v26=9 and no tunneling doubling is resolved for any of the transitions. These progressions show that the tunneling energy level splitting of tropolone in the vibrationless level of the ground electronic state can be at most...

Infrared spectra of the H2F2 dimer isolated in Ar, N2, CO, and several mixed matrices, are reported and values for all six fundamental vibration frequencies are given for the Ar‐isolated dimer. Evidently due to molecular nonrigidity, the relative intensity of ν1 and ν2 of the H2F2 dimer depend strongly on the matrix environment. The HF stretching fequencies for X‐‐‐HF bimolecules isolated in Ar, with X=Xe, N2, CO, and H2O are observed. Matrix isolated cyclic trimer and higher oligomers of HqFq are observed to have their HF stretching bands in the 3430 to 3175 cm−1 region. The discussion includes a vibrational predissociation lifetime correlation for the vapor phase dimer.

The fluorescence excitation spectrum of the A 1 B 2 − X 1 A 1 transition of tropolone was investigated as a function of pulsed laser intensity, I . For 0.5 I 2 the weak fluorescence intensity was found to increase approximately as aI + bI 2 . The band profiles, which showed no evidence of resolvable rotational fine structure, were modeled by computations assuming a Lorentzian lineshape for the individual rotational transitions.

Infrared matrix-isolation spectra are reported for the tropolone monomers C 7 H 5 O 2 H and C 7 H 5 O 2 D. Tropolone possesses an intramolecular hydrogen bond and the possibility for proton tunneling from one oxygen atom to the other. Clearcut multiplets attributed to tunneling are observed for the OD stretching mode and for heavy atom modes of both C 7 H 5 O 2 D and C 7 H 5 O 2 H. The (mixed) carbonyl stretching mode particularly seems to facilitate tunneling from one conformation to the other. The tunneling phenomena suggests that tropolone monomer has nearly C 2 v symmetry. As a crude estimate, the tunneling potential energy barrier is calculated to be less than 5600 cm −1 in the ground electronic state of C 7 H 5 O 2 D. The barrier is lower in the Π ∗ electronic state than in the ground electronic state. A vibrational assignment that encompasses most of the 39 fundamental modes is proposed. The vibrations are classified using C 2 v symmetry species and a parallel vibrational analysis is presented for tropone, C 7 H 6 O, which is a true C 2 v molecule.

Infrared spectra have been obtained for 18 O and D isotopically enriched trifluoroacetic acid monomers isolated in argon matrices. The 18 O isotope effect is particularly clear for the nominal COH angle bending vibration. The new data are combined with literature results on several related CF 3 -containing molecules to present a self-consistent compilation of vibrational assignments for the entire group. The basic molecules are CF 3 COOH, (CF 3 CO) 2 O, CF 3 CHO, CF 3 CFO, CF 3 CClO and CF 3 COSH.

Infrared spectra of monomeric acetic acids [CH3COOH(D) and CD3COOH(D)] isolated in Ar and N2 matrices near 4°K are reported. A large anharmonic potential‐energy contribution is evident from the observed isotopic frequency shifts. It is proposed that this arises primarily from the double‐minimum potential‐energy curve for tunneling of the H atom between O atoms via the COH angle bending coordinate. In all cases, important coupling between the COH angle bending and C–O stretching coordinates is observed, with a strong addition of CD3 (umbrella) angle deformation in the case of CD3COOH. Exceptional matrix effects are observed for the coupled vibrations of the –OH molecules. A complete assignment is proposed for these acetic acids on the basis of the new data. As additional supporting evidence, the spectrum of matrix‐isolated CH3COF and of acetic acid vapor at low pressure and long path length are reported.

The infrared spectra of SbF5 and AsF5 suspended in argon and neon matrices have been recorded between 4000 and 100 cm−1. The spectra of SbF5 and AsF5 are radically different and show that monomeric SbF5 belongs to symmetry group C4υ rather than to group D3h. The spectral effects induced by forming solid SbF5 from the monomers are understood in terms of associations through Sb–F–Sb fluorine bridge bonds. The behavior is compared with that observed previously for SF4. No evidence for fluorine bridging occurs with AsF5.

Spectra of matrix-isolated trifluoroacetic acid species and matrix-isolated and solid trifluoroacetic anhydride are reported. Vibrational assignments are given for the CF3COOH and CF3COOD monomers, the i.r. active vibrations of (CF3COOH)2 and (CF3COOD)2 cyclic dimers, and (CF3CO)2O. The analysis suggests that a very low barrier hinders internal rotation of the CF3 group in the acids and also indicates the probable absence of an equilibrium plane of symmetry in the monomers. The truth of these statements hinges on the interpretation of the data for the nominal COH bending vibration. For CF3COOH monomer this band shows two components that are separated by ~15 cm−1, while CF3COOD monomer yields a single, sharp band that is red-shifted by a normal amount. These observations, coupled with the anomalous isotopic frequency behavior found for this vibration with acetic acid monomers [C.V. Berney, R.L. Redington and K.C. Lin, J. Chem. Phys.53, 1743 (1970)], reinforces the suggestion that a relatively low barrier hinders H atom transfer between the two O atoms via a coordinate that originates as COH angle bending.

The infrared spectra of matrix‐isolated ClF3, BrF3, and BrF5 have been investigated in the region from 800 to 33 cm−1. All six fundamentals have been observed in Ar and N2 matrices for ClF3 and BrF3, which are planar T‐shaped molecules, and most of them in a Ne matrix. The two lowest‐frequency fundamentals, which could not be separated in the gas‐phase spectra [H. Selig, H. H. Claassen, and J. H. Holloway, J. Chem. Phys. 52, 3517 (1970)] are clearly visible in the matrix‐isolation spectra. Both molecules show only small gas‐matrix frequency shifts, and multiplet structure for both molecules suspended in Ar matrices may be understood in terms of isotope and matrix site effects. Several absorption bands found at low matrix‐isolation ratios are attributed to dimers and a tentative partial vibrational assignment is presented. Similar dimer structures are assumed for both substances. The structure seems best represented with two F bridges formed using the long‐bond F atoms. The structure is favored by the x‐ray and NMR data. Re‐evaluation of the spectral data for SF4, and comparison with ClF3 suggests reversing the literature assignments of the antisymmetric axial and equatorial S–F stretching vibrations for the SF4 monomer. Then consistent experimental agreement between the closely related ClF3 and SF4 molecules ensues and analogous structures for the matrix‐isolated dimers follow. Considered as transient liquid‐state species, the dimers demonstrate a highly likely fluorine exchange mechanism involving an intermolecular transfer of the bridging F atoms that can account for the exchange of nonequivalent F atoms which is observed in the liquid state for these systems. The vibrational frequencies of ClF3, SF4, BrF3, and BrF5 are compared and force constants are calculated for ClF3 and BrF3.

Infrared spectra of LiF species suspended in solid Ne, O2, CO, and CH4 at liquid‐helium temperature have been obtained. The monomeric LiF frequency (in cm−1) is 869 in Ne, a doublet at 819–824 in O2, 787 in CH4, and a doublet at 710–725 in CO, compared to 895 cm−1 in the gas phase. Some of these values seem consistent with Linevsky's statement that the solvent shift in LiF is primarily of electrostatic origin. Electrostatic calculations were made for the rare‐gas matrices in order to observe the effects of asymmetric solute positions in the ideal lattice substitutional site and to test ideal lattice breakdown, or ``solvation'' of the monomeric LiF solute. For reasonable distance parameters, both types of calculation can account for most, or all, of the observed solvent shift. These simple calculations show ways to reduce the dispersive contributions to commonly observed values but likewise illustrate the presently unsatisfactory state of a genuine understanding of the LiF—matrix systems.

High‐resolution spectra of H2O, HDO, and D2O suspended in solid Ar have been observed in the range 4°—27°K. Evidence has been obtained for the rotation of these species in Ar matrices. The majority of the observed lines are found to be in very good agreement with low‐J transitions of the gas spectra, allowing for a small matrix shift. Transitions arising from both spin modifications of H2O and D2O have been observed and the observed intensities suggest that a large degree of spin equilibration has taken place. A small contamination of O2 may be responsible for this. A perturbation of one of the rotational levels of ν2—H2O is discussed.

The infrared spectrum of SF4 trapped in an argon matrix at ∼4°K shows characteristic changes when the sample is warmed to allow diffusion and recooled. These changes are discussed in terms of association of the SF4 molecules, and their simplicity and stability suggest a simple model for the dimer and more highly associated species. Weak bonding appears to exist between the pair of equatorial F atoms on one SF4 molecule and the sulfur lone pair of electrons on an adjacent SF4 molecule. The spectrum of SOF2, present as an impurity, shows changes suggestive of the formation of an SF4–SOF2 complex. The observations, together with the NMR data of Muetterties and Phillips indicate that the mechanism for the rapid exchange of axial and equatorial fluorines in liquid SF4 at room temperature is not intermolecular. A low barrier for the inversion of isolated SF4 is also ruled out by the apparent absence of inversion effects in the microwave spectrum of the vapor, studied by Tolles and Gwinn. The conclusion is drawn that there are two possible paths for the dimerization or association reactions, and that one of these paths involves inversion of the SF4 molecule. The activation energy of the latter reaction corresponds closely to the inversion energy of SF4, which must be markedly smaller in this dimeric transition state than in an isolated SF4 molecule.

Five of the lowest frequency species a1, b1, and b2 vibrational modes of tropolone in the A 1B2 electronic state are studied using high resolution laser fluorescence excitation spectroscopy of the jet‐cooled sample. The ν’26(b1) mode at 38 cm−1, most probably a ‘‘folding’’ vibration of the seven‐membered and O⋅⋅⋅HO chelated ring systems of tropolone, is observed in the progression 26v0 with primarily even valued quantum numbers to v26=14. Tunneling doublets in the progression are observed to decrease from 18.90 cm−1 in the vibrationless state to unresolvable values when v’26 is larger than 8, thereby demonstrating highly mode‐specific quenching of the ‘‘H atom’’ tunneling process by these low frequency, out‐of‐plane, ring deformation vibrations. The progression 251026v0 is observed to v26=9 and no tunneling doubling is resolved for any of the transitions. These progressions show that the tunneling energy level splitting of tropolone in the vibrationless level of the ground electronic state can be at most...