Index of content:
Volume 85, Issue 10, 15 November 1986

Isomerization and unimolecular dissociation channels of the oxalic acid monomer
View Description Hide DescriptionThe results of molecular orbital calculations performed for the oxalic acidmonomer using the 6‐31G, 6‐31G*(5d), 6‐31G**(5d), and MP2/6‐31G*(5d) levels are reported. At the latter three levels of approximation the geometry with intramolecular hydrogen bonds is calculated to have the lowest energy, and the energies calculated for planar rotational conformers fall within about 2 kcal/mol of one another. The barrier hindering internal rotation of the carboxyl groups about the C–C bond for the nonhydrogen bonded conformer is calculated to be less than 1 kcal/mol, and secondary potential energy wells for g a u c h e, rather than for the expected c i s, carbonyl orientation are found about 300 cal/mol above the t r a n s conformer at the 6‐31G level. Vibrational frequencies calculated at the 6‐31G level for the H‐bonded conformer average about 9% higher than the observed values. On including electron correlation at the MP2 level the barrier calculated for the concerted, symmetrical, transfer of two protons between equivalent potential energy minima is 31.9 kcal/mol. This value is probably an upper bound. When electron correlation is ignored, calculations for the transfer of a single proton lead to a potential energy well that stabilizes an (HO)_{2}CCO_{2} configuration with C _{2v } symmetry at an energy 26–28 kcal/mol above that of the H‐bonded ground state conformer. When electron‐correlation is included at the MP2 level there is a saddle point in the PES instead. This is the transition state for proton exchange by successive transfers, and a search for its position at the MP2 level was not made. However, energy values of 25.7 and 36.6 kcal/mol are calculated using MP2/6‐31G*(5d) at optimized 6‐31G critical point geometries, and these values are taken as temporary estimates for the MP2 level transition state energies for two‐step proton exchange and for decarboxylation, respectively. It remains to be determined whether the stepwise proton exchange channel will be present for higher level calculations. The results of these MO calculations are consistent with the experimental observations of Lapidus, Barton, and Yankwich
yielding first order kinetics, a low activation energy (31.5 kcal/mol), and complex kinetic isotope effects for the thermal decarboxylation of the oxalic acidmonomer. Because of its importance in this reaction, calculations of the 1,2 H atom shift of dihydroxycarbene are also reported.

Spin uncoupling in the 6s Rydberg states of methyl iodide. Rotational subband structure in two‐photon resonant absorption
View Description Hide DescriptionRotational envelopes of some of the lowest‐energy (R6s) Rydberg transitions [i.e., the two electronically degenerate B ^{3} E(1) and C ^{1} E(1) band systems and the totally symmetric A _{1}(0^{+}) band system] of methyl iodide are calculated for a two‐photon absorption process and compared to the experimental spectra obtained using the technique of two‐photon resonant multiphoton ionization (MPI). The rotational band shapes, which were computed with molecular constants extracted from a simulation of high‐resolution, one‐photon absorption data, reproduce all significant features of the experimental MPI spectra, including temperature and polarization dependencies. Since one‐ and two‐photon processes are governed by different selection rules, the present results are an independent verification of spin uncoupling in these R6s states. These results, therefore substantiate the conclusion that the R6s electron uncouples from the molecular frame during the evolution of the Rydberg state and that, consequently, the molecular constants extracted from experimental band shape analysis pertain to the freely rotating, electronically degenerate, molecular–ion core.

A theoretical analysis of the main effects in magnetic vibrational circular dichroism
View Description Hide DescriptionAn analysis is made of all the lowest order contributions to magnetic vibrational circular dichroism (MVCD). Three different contributing effects are found and characterized and are interpreted in terms of the interference and interaction of induced electronic currents.

C–H local modes in cyclobutene. II. Laser photoacoustic studies 10 000–17 000 cm^{−} ^{1}. Vibrational structure and C–H local mode dynamics
View Description Hide DescriptionIn part I of this study [Baggott, Clase, and Mills, Spectrochim. Acta Part A 4 2, 319 (1986)] we presented FTIR spectra of gas phase cyclobutene and modeled the v=1–3 stretching states of both olefinic and methylenic C–H bonds in terms of a local mode model. In this paper we present some improvements to our original model and make use of recently derived ‘‘x,K relations’’ to find the equivalent normal mode descriptions. The use of both the local mode and normal mode approaches to modeling the vibrational structure is described in some detail. We present evidence for Fermi resonance interactions between the methylenic C–H stretch overtones and ring C–C stretch vibrations, revealed in laser photoacoustic spectra in the v=4–6 region. An approximate model vibrational Hamiltonian is proposed to explain the observed structure and is used to calculate the dynamics of the C–H stretch local mode decay resulting from interaction with lower frequency ring modes. The implications of our experimental and theoretical studies for mode‐selective photochemistry are discussed briefly.

Quantum defect analysis of HD photoionization
View Description Hide DescriptionA multichannel quantum defect calculation is shown to reproduce most observed features in several portions of the HD photoabsorption spectrum. The rovibrational frame transformation theory of Atabek, Dill, and Jungen is reformulated in terms of a quantum defect matrix. The calculation accounts for spectral regions far from dissociation thresholds despite its neglect of g–u mixing.

Neon matrix ESR and CI theoretical investigation of ^{1} ^{0}BF^{+} and ^{1} ^{1}BF^{+}: Photoionization of BF from reactive laser sputtering and high temperature sources
View Description Hide DescriptionThe ^{1} ^{1}BF^{+} and ^{1} ^{0}BF^{+} radical cations have been generated and trapped in neon matrices at 4 K using the combined techniques of pulsed reactive laser sputtering and photoionization at 16.8 eV. An independent high temperature source of BF(g) in conjunction with photoionization was also employed and a comparison between these two different generation methods reveals no significant differences in their ESR spectra. The magnetic parameters for ^{1} ^{1}BF^{+} measured in neon matrices are g _{∥}=2.0012(3), g _{⊥}=2.0004(3), (^{1} ^{1}B) A _{∥}=1784(1), and A _{⊥}=1727(1) MHz, (^{1} ^{9}F)A _{∥}=410(1), and A _{⊥}=152(1) MHz. Extensive a b i n i t i otheoretical calculations have been conducted for BF^{+} and the nuclear hyperfineproperties obtained from various types of CI wave functions show excellent agreement with the experimental measurements. Valence orbital occupancies obtained from a Mulliken‐type population analysis performed on the CI wave functions are compared with the conventional free atom comparison method for obtaining electronic structure information from ESRAtensor measurements. The distribution of the unpaired electron in BF^{+} is compared to six other isoelectronic first row radicals.

Spectra and structure of small ring molecules. XLVII. Rotational and vibrational spectra and conformational stability of cyclobutylcarboxylic acid fluoride
View Description Hide DescriptionThe microwave spectrum of cyclobutylcarboxylic acid fluoride, c‐C_{4}H_{7}CFO, has been recorded from 18.0 to 40.0 GHz. The a‐type R‐branch transitions have been observed and assigned for the ground and two vibrationally excited states of the asymmetric torsional mode as well as for the first vibrationally excited state of the ring puckering fundamental. The rotational constants were determined from the frequency fit to a rigid rotor model for the ground vibrational state to be: A=5467.35±0.03, B=1884.76±0.01, and C=1558.64±0.01 MHz. These constants are shown to be consistent with an equatorial‐g a u c h econformation (i.e., the CFO group is in the equatorial position relative to the ring and the C=O bond is eclipsing, or nearly so, one of the C–C bonds of the ring). From relative intensity measurements the frequency for the asymmetric torsion for this conformer is estimated to be 72±10 cm^{−} ^{1}. From the Stark effect the dipole moment components were determined to be: ‖μ_{ a }‖ =2.97±0.02, ‖μ_{ b }‖ =0.84±0.01, ‖μ_{ c }‖ =0.35±0.01, and ‖μ_{ t }‖ =3.11±0.01 D. The central line of an additional conformer has been identified with a B+C value of 3482 MHz which is consistent with the expected value for the equatorial‐t r a n s conformer. The infrared (3500 to 30 cm^{−} ^{1}) and Raman spectra (3200 to 30 cm^{−} ^{1}) have also been recorded for the gaseous and solid states of cyclobutylcarboxylic acid fluoride.
Additionally, the Raman spectrum of the liquid phase has been recorded and qualitative depolarization values have been obtained. From the relative intensities of the Raman lines of the gas at 918 cm^{−} ^{1} (equatorial‐t r a n s) and 926 cm^{−} ^{1} (equatorial‐g a u c h e) as a function of temperature, the enthalpy difference was found to be 352±100 cm^{−} ^{1} (1.01 kcal/mol) with the equatorial‐g a u c h e being more stable. A complete vibrational assignment is proposed based on infrared band contours, depolarization values, and group frequencies. These results are compared to similar quantities for some related molecules.

Reduction of 1+1 resonance enhanced MPI spectra to populations and alignment factors
View Description Hide DescriptionA theory is presented to reduce 1+1 resonance enhanced multiphoton ionization (REMPI) spectra to accurate rovibrational state population distributions. Classical and quantum mechanical treatments are developed to model the polarization dependence of the REMPI signal from an initially aligned ground state having cylindrical symmetry. The theory includes the effects of saturation and intermediate state alignment. It is demonstrated that, for favorable cases, 1+1 REMPI allows the determination of the relative population as well as the quadrupole and hexadecapole moments of the alignment for rovibrational levels of a linear molecule. The classical treatment differs from that of the quantum treatment by less than 5% for rotational quantum numbers greater than J=4, suggesting that the classical treatment suffices for 1+1 REMPI in most molecular systems.

Reduction of 1+1 resonance enhanced MPI spectra to population distributions: Application to the NO A ^{2}Σ^{+}–X ^{2}Π system
View Description Hide DescriptionA two‐step methodology is presented for extracting ground state population distributions and alignment factors from 1+1 resonance enhanced multiphoton ionization (REMPI) spectra. In the first step the ion signal is corrected for variation with laser intensity as it is collected, generating an isopower spectrum. In the second step populations and alignments are derived from the isopower spectrum by correcting for the interdependent effects of saturation and intermediate state alignment. This procedure is applied to a room temperature thermal distribution of nitric oxide using the 1+1 REMPI process in which lines of the NO A ^{2}Σ^{+}–X ^{2}Π (0,0) band constitute the resonant transition. The present treatment is able to recover the known rovibrational population distribution, independent of branch choice, over a wide range of practical operating conditions.

Temperature dependence of ^{7} ^{7}Se, ^{1} ^{2} ^{5}Te, and ^{1} ^{9}F shielding and M‐induced ^{1} ^{9}F isotope shifts in MF_{6} molecules
View Description Hide DescriptionThe high resolution NMRspectra of ^{7} ^{7}Se, ^{1} ^{2} ^{5}Te, and ^{1} ^{9}F nuclei in SF_{6}, SeF_{6}, TeF_{6}, and WF_{6} molecules are measured as functions of density and temperature in the pure liquid and in the dilute gas. The ^{1} ^{9}F chemical shifts between the isotopomers of these molecules are observed in all but WF_{6}, and the ^{ m’}MF_{6}–^{ m }MF_{6} shifts exhibit a strict proportionality to the mass factor (m’–m)/m’.

Rovibrational averaging of nuclear shielding in MX_{6}‐type molecules
View Description Hide DescriptionCalculations of the mean M–X bond displacements in octahedral MX_{6} molecules by the Ltensor and Bartell methods using anharmonic force fields for SF_{6}, SeF_{6}, TeF_{6}, WF_{6}, PtCl^{=} _{6}, and PtBr^{=} _{6} are compared with electron diffraction data in SF_{6}, and are used in the interpretation of ^{1} ^{9}F, ^{7} ^{7}Se, ^{1} ^{2} ^{5}Te, and ^{1} ^{9} ^{5}Pt chemical shifts in these molecules. The temperature and mass dependence of M and X chemical shifts can be written in terms of 〈Δr _{MX}〉, and the two together provide a critical test of anharmonic force fields. The direct proportionality of the isotope shifts to the mass factor (m’−m)/m’ is found to be a direct consequence of the calculated linear dependence of 〈Δr〉−〈Δr〉’ and 〈(Δr)^{2}〉−〈(Δr)^{2}〉’ on this mass factor. The observed isotope shifts and temperature dependent chemical shifts in the zero pressure limit can be used to determine the sensitivity of the nuclear magnetic shielding to bond extension.

Reactive collision dynamics by far wing laser scattering: Mg+H_{2}
View Description Hide DescriptionWe have measured the far wing absorption profiles of the MgH_{2} collision system leading to both the nonreactive formation of Mg* and into two distinct final rotational states of the reaction product MgH (v‘=0, J‘=6, 23). We have observed qualitatively expected behavior including a pronounced red wing in the reactive absorption profile indicating strong reaction probability on the excited attractive potential surfaces. We have also observed novel aspects of the excited state dynamics including reactive vs nonreactive channel competition effects and a strong far blue wing reactive absorption suggesting significant reaction probability even for trajectories on the repulsive surfaces. We have developed a simple theoretical model to semiquantitatively explain our experimental results. This model uses standard quasistatic theory to estimate the absorption probability as a function of detuning between levels of MgH_{2} and with assumed nonreactive vs reactive branching ratios, accounts for the subsequent evolution on the excited potential surfaces. This theory correctly predicts the overall shapes of the profiles and in general gives reasonable predictions for the relative magnitudes of the wing intensities.

Reflection optical activity of uniaxial media
View Description Hide DescriptionThe possibility to investigate optical activity of uniaxial media by measuring the difference between the intensities of reflected electromagnetic waves (emw) of different circular polarization (reflection optical activity—ROA) is discussed. The components of the gyration tensor and the polarization of the normal emw in uniaxial media are found. The reflection of emw is studied in two cases: (a) optical axis normal to the boundary; (b) optical axis normal to the plane of incidence. Three modifications of differential spectroscopy are proposed: (1) left and right circularly polarized waves—usual reflection; (2) total reflection–circular polarization (TROA); (3) total reflection of linearly polarized waves with different azimuths of vibration. The specific features of the proposed methods (ROA and TROA) are discussed as well as the possibility of studying the optical activity induced by nondegenerate and degenerate excitations (vibrations).

The microwave spectrum of the K=0 states of Ar–NH_{3}
View Description Hide DescriptionThe microwave spectrum of Ar–NH_{3} has been obtained using molecular beam electric resonance spectroscopy and pulsed nozzle Fourier transformmicrowave spectroscopy. The spectrum is complicated by nonrigidity and most of the transitions are not yet assigned. A ΔJ=1, K=0 progression is assigned, however, and from it the following spectroscopic constants are obtained for Ar–^{1} ^{4}NH_{3}: (B+C)/2=2876.849(2) MHz, D _{ J } =0.0887(2) MHz, e q Q _{ a a } =0.350(8) MHz, and μ_{ a } =0.2803(3) D. For Ar–^{1} ^{5}NH_{3} we obtain (B+C)/2 =2768.701(1) MHz and D _{ J } =0.0822(1) MHz. The distance between the Ar atom and the ^{1} ^{4}NH_{3} center of mass R _{CM} is calculated in the free internal rotor limit and obtained as 3.8358 Å. In the pseudodiatomic approximation, the weak bond stretching force constant is 0.0084 mdyn/Å which corresponds to a weak bond stretching frequency of 35 cm^{−} ^{1}. The NH_{3} orientation in the complex is discussed primarily on the basis of the measured dipole moment projection and the quadrupole coupling constant. It is concluded that the Ar–NH_{3}intermolecular potential is nearly isotropic and that the NH_{3} subunit undergoes practically free internal rotation in each of its angular degrees of freedom. Spectroscopic evidence is presented which indicates that the NH_{3} subunit also inverts within the complex. These conclusions concerning the internal dynamics in the Ar–NH_{3} complex support the model initially proposed in our previous study of the microwave and infrared spectra of this species.

Pure rotational spectrum of SnH_{4} in the vibrational ground state observed by infrared‐radio frequency double resonance
View Description Hide DescriptionPure rotational Q‐branch transitions caused by the centrifugal‐distortion‐induced dipole moment were observed in the vibrational ground state of SnH_{4} by infrared‐radio frequency double resonance using a tunable diode laser. Forty‐nine double resonance signals were observed for 15 rotational transitions with 6≤J≤20 for seven isotopic species of Sn. The isotope effects on the transition frequencies were found to be negligible to the experimental accuracy of a few tens of kilohertz. The tensor centrifugal distortion constants were determined as follows: D _{ t } =58 398.6±7.0 Hz, H _{4t } =−4.297±0.046 Hz, and H _{6t } =2.340±0.016 Hz.

Absolute intensities of infrared‐active fundamentals and combination bands of gaseous PuF_{6} and NpF_{6}
View Description Hide DescriptionResults are presented for the absolute integrated molar absorption coefficients of the infrared‐active fundamental and binary combination bands of PuF_{6} and of NpF_{6} between 1400 and 450 cm^{−} ^{1}. The low resolution band contours of the combination bands ν_{1}+ν_{3}, ν_{2}+ν_{3}, ν_{3}+ν_{5}, ν_{2}+ν_{6}, and ν_{2}+ν_{4} are discussed. From band center positions, the six harmonic wave numbers were obtained and Coriolis constant ζ_{ i } (i=3, 4, 5, and 6) estimated. For PuF_{6}, the six normal mode vibrations appear to group into one ‘‘local mode’’ stretching at a mean value of 589±50 cm^{−} ^{1} and one local mode bending at 195±5 cm^{−} ^{1}. The role of the heavy central atom and the magnitudes of the vibrational force constants are discussed in comparison with UF_{6}. The measured intensity for ν_{3} is discussed in comparison with previously predicted values.

Constant ionic state spectroscopy of N_{2}O. Dispersed fluorescence as a probe of molecular autoionization
View Description Hide DescriptionWe report electronic autoionization studies of N_{2}O using vibrationally resolved constant ionic state (CIS)spectroscopy. Vacuum ultraviolet synchrotron radiation is the excitation source, and we determine the relative partial photoionization cross‐section curves for alternative vibrational levels (v’) of the ion by detecting dispersed fluorescence [N_{2}O^{+}(A ^{2}Σ^{+},v’→X ^{2}Π,v‘)] from the ion. Excitation spectra sampling different vibrational levels reveal significant changes in the 3pπ resonance profile, including shifts of the resonance minima, and previously unobserved features. Analysis of the v’=(0,1,0) CIS spectra demonstrate that this level of the ion is produced predominantly by photoionization of the target molecules in the (0,1,0) level, i.e., via hot band excitation. These results are discussed in detail, as well as possible extensions and further studies.

Wavelength dependent photoelectron spectra of rotationally resolved autoionizing levels of nitric oxide
View Description Hide DescriptionPhotoelectron spectra obtained following two photon excitation to rotational levels of the 9dσπ, v=2 state, which can autoionize into the NO^{+} v ^{+}=0 and 1 continua, are presented. The photoelectron spectra obtained along the laser polarization axis show a distinct wavelength dependence across the rotationally resolved autoionizing resonances; the angular distributions vary dramatically, becoming more isotropic on resonance. The branching ratio into the NO^{+} v ^{+}=1 state increases significantly on the resonance, in accord with the Δv=−1 propensity rule for vibrational autoionization.

Magnetic interactions in HCF and HSiF studied by sub‐Doppler spectroscopy
View Description Hide DescriptionThe Ã ^{1} A‘(000)–X̃ ^{1} A’(000) band has been observed for HCF and HSiF at sub‐Doppler resolution using the intermodulated fluorescence (IMF) technique. The hyperfine structure due to the ^{1} ^{9}F nuclear spin/overall rotation interaction has been resolved for HCF, yielding the following coupling constants: C ^{‘} _{ a a } =4.19(14) and C _{ a a } =−7.71(20) MHz with one standard error in parentheses for the X̃ and Ã states, respectively. In contrast, no hyperfine structure has been observed for HSiF. The Zeeman effect has also been examined for both species by IMF, and the following rotational g factors have been derived: g ^{‘} _{ a a } =−5.69(37)×10^{−} ^{3} and g _{ a a } =(5–10)×10^{−} ^{3} for X̃ and Ã of HCF, respectively, and g ^{‘} _{ a a } =−1.47(9)×10^{−} ^{3} and g _{ a a } =1.57(11)×10^{−} ^{3} for X̃ and Ã of HSiF, respectively, all in the unit of Bohr magneton with one standard error in parentheses. The two types of molecular constants thus determined have been discussed in terms of electronic Coriolis interaction between the X̃ and Ã states in each of the two molecules.

Two‐photon circular dichroism in lanthanide (III) complexes
View Description Hide DescriptionThe theory of two‐photon circular dichroism in lanthanide (III) systems was developed within the crystal field model. From the selection rules for the process four different types of transitions were distinguished and their dissymmetry factors were estimated.