J.L. Duncan

Infrared crystal spectra of C2H4, C2D4, and as-C2H2D2 and the general harmonic force field of ethylene

Abstract Carbon-13 frequency shifts for C 2 H 4 , C 2 D 4 , and as -C 2 H 2 D 2 have been measured in isotopic solid solutions in crystalline films at 60 K. All but two of the shifts (for as -C 2 H 2 D 2 ) are compatible with recently determined ζ data for C 2 H 4 , with 13 C frequency shifts for C 2 H 4 and C 2 D 4 in the gas phase and with conventional frequency data. Together, these data completely determine with precision all 18 parameters of the GHFF for ethylene, the previous ambiguity in choice between two sets of A g species force constants being removed. The force field reproduces closely the observed centrifugal distortion constants for C 2 H 4 , a ζ constant observed for trans -C 2 H 2 D 2 , and the inertia defects for C 2 H 4 , C 2 D 4 , and as -C 2 H 2 D 2 . Vibration and rotation constants for all isotopically deuterated ethylenes are calculated. Possible explanations for the two anomalous crystal shifts in as -C 2 H 2 D 2 involve the effects of the crystal field, and failure of the use of Dennisons rule for making anharmonic corrections to the shifts. The former explanation is preferred as a result of thorough analysis of the anharmonicity constants for as -C 2 H 2 D 2 determined from many overtone and combination bands in the gas and crystal spectra.

The ground-state average and equilibrium structures of formaldehyde and ethylene

The use of recently determined highly precise rotational constants for various isotopes of formaldehyde and ethylene enables the ground-state average structures to be determined to high accuracy. Harmonic corrections to the inertial constants are calculated using recently determined general harmonic force fields. In the case of formaldehyde, the data enable changes in all three structural parameters due to isotopic mass effects to be determined. For ethylene, only the change in CH bond length on deuteration is well characterized, due to the somewhat lower accuracy in the rotational constants. Using isotopic structural differences transferred from formaldehyde, and supported by calculations, ground-state rotational constants for all deutero isotopes of ethylene are predicted to what is considered to be better than 0·1 per cent reliability. The change in CH bond length on deuteration in ethylene is less than in formaldehyde. This is supported by calculations, although it appears also that the CH bond stretc...

The ground state structures of disilane, methyl silane and the silyl halides, and an SiH bond length correlation with stretching frequency

Abstract Infrared spectra of the molecules Si 2 HD 5 , SiHD 2 CH 3 , SiH 3 CHD 2 , SiHD 2 X (X = F, Cl, Br, I) have been recorded at 0.05 cm −1 resolution. Analyses of “perpendicular” fundamentals of these very slightly asymmetric top molecules permit the accurate determination of their ground state ( A 0 — B 0 ) constants, from which the A 0 rotational constants may be obtained. When combined with all known Raman and microwave constants, this enables the ground state structures to be determined with precision, allowance being made for the shortening of SiH(CH) bonds on deuteration. For disilane this work represents the first spectroscopic determination of a complete structure, while for the silyl halides it demonstrates overestimates in the SiH bond length of nearly 1% by previous workers. In the case of methyl silane, comparison with a recent ab initio structure calculation reveals a considerable overestimate of the CSi bond length in the latter. A correlation between isolated SiH stretching frequency and ground state bond length over 15 molecules reveals a somewhat reduced sensitivity compared with the corresponding CH correlation. Nevertheless, a predictive capability of better than ±0.003 A seems possible from the isolated SiH stretching frequency, at least for molecules with fully saturated bonding.

The infra-red spectrum of 13CH3F and the general harmonic force field of methyl fluoride

Analyses of the v 3, 2v 3, and (predominantly) v 1 parallel bands, and of the v 4 and v 6 perpendicular fundamentals have been made for 13CH3F in terms of the rotational structure observed with a resolution of ∼0·2 cm-1. In addition, the band centres of the strongly Coriolis-interacting v 2 and v 5 fundamentals are accurately located. Some elucidation of the complex Fermi resonance interactions in the 3000 cm-1 region is achieved through study of spectra of crystalline samples. This enables all three components of the v 1, 2v 2, 2v 5 0 triad to be observed for both 12CH3F and 13CH3F, and estimates to be made for the unperturbed vibration frequencies. The 13C frequency shifts determined for all six fundamentals are used in conjunction with existing frequency, Coriolis ζ, and centrifugal distortion data for CH3F, CD3F, CHD2F and CH2DF, to determine the general harmonic force field for methyl fluoride. The extra shift data enable all 12 parameters of the force field to be fixed within narrow limits for the f...

13C frequency shifts and the general harmonic force fields of methyl chloride, bromide and iodide

General harmonic and hybrid orbital force fields have been calculated for methyl chloride, bromide and iodide using the best available frequency, Coriolis coefficient, and centrifugal distortion data, including the recent 13C frequencies for the chloride and iodide. Anharmonicity corrections were made to the fundamental frequencies using Dennisons rule and x values of 0·04 for CH stretching, 0·02 for CH bending, and 0·01 for CX stretching modes. These were the most reasonable values which did not seriously over-compensate for the product rule deficiencies in the 12C and 13C frequencies. The 13C and CD3X frequency information was input in the form of frequency shifts from the 12CH3X frequencies. All 12 parameters in the most general harmonic force fields are determined with significance for the first time, although the previously uncertain parameters F12 and F13 are only determined with good precision when the 13C frequency data are utilized in addition to the other information. The signs of F12, F45 and ...

An improved general harmonic force field for ethylene

Abstract The recent availability of certain critical experimental data, both from this and other laboratories, for isotopic species of the ethylene molecule, has enabled the previously published GHFF to be redetermined with much greater precision. Several significant changes in the force constants occur, bringing the empirical values into better overall agreement with ab initio values. Calculated data using the new force field are considered to be sufficiently reliable to be used with confidence in spectroscopic and structural applications and are listed.

High-resolution infrared spectrum and rotational constants of ethylene-H4

Abstract Four bands in the infrared spectrum of ethylene-H 4 are studied with a resolution of 0.03 – 0.04 cm −1 , the A -type ν 11 fundamental, the B -type ν 9 fundamental, and the B -type ν 5 + ν 12 and ν 6 + ν 11 combination bands. From ∼300 combination differences, the following ground-state rotational constants are determined: A 0 = 4.86596 ± 0.00018, B 0 = 1.001329 ± 0.000061, C 0 = 0.828424 ± 0.000062, D J 0 = 1.447 ± 0.070 × 10 −6 , D JK 0 = 1.468 ± 0.063 × 10 −5 , D K 0 = 9.17 ± 0.16 × 10 −5 cm −1 . Upper-state rotational constants are reported for all four bands, some Coriolis interactions are identified, and the relevant ξ constants are determined. The value of the band center of ν 5 + ν 12 leads to a suggested reassignment of Q branches in the Raman active fundamental ν 5 , and a revision of the band center to 3086 cm −1 . The new data for C 2 H 4 determined in this work are summarized in Table VII, along with all the other currently available data on the vibrational and rotational constants.

Methyl cianide: Spectroscopic studies of isotopically substituted species, and the harmonic potential function

Abstract The infrared gas-phase spectra of CH 3 CN, 13 CH 3 CN, CH 3 13 CN, CH 3 C 15 N, CD 3 CN, and CD 3 13 CN have been studied in detail, in order to determine accurately the fundamental vibration frequency displacements on heavy isotopic substitution. A number of important Fermi resonances have been identified, and treated quantitatively. The unperturbed fundamental frequencies and heavy isotopic displacements form a self-consistent set of data, which, together with Coriolis zeta and centrifugal distortion constants, enable the harmonic potential function of methyl cyanide to be determined with only one constraint. A comparison between the latter and results from an ab initio calculation reveals disagreement in the values of two interaction constants, which seem well outside our experimental error. Infrared frequencies in crystalline films of CD 3 CN and CD 3 13 CN at 78 K are also reported.

Ground state rotational constants of H2CCD2 and C2D4 and geometry of ethylene

Abstract The ν6 and ν10 B-type and the ν7 and ν8 C-type fundamentals of H2CCD2 have been studied at a resolution of ∼0.2 cm−1, and the A-type ν1 fundamental at a resolution of 0.03–0.04 cm−1. From analyses of the rotational structure in these bands, accurate values of the ground state rotational constants A0, B0, and C0 are obtained. The band centers of the very weak B-type bands are accurately located for the first time. For C2D4, values of A0, B0, and C0 are determined from the ground state combination differences obtained from the rotational Raman data of Dowling and Stoicheff and the infrared data of Allen and Plyler. For both molecules, the experimental and theoretical inertia defect values are in close agreement. The sets of ground state rotational constants for H2CCD2 and C2D4 are combined with those for C2H4 of a previous study to yield for the ground state geometry of ethylene: r CH 0 = 1.085 ± 0.002 A , r CC 0 = 1.339 ± 0.001 A , αHCH0 = 117°50′ ± 15′.

Microwave spectra of deuterated ethanes: Internal rotation potential function and rz structure

Abstract The microwave spectra of CH 3 CHD 2 in the first excited torsional state and of CH 3 CH 2 D and CD 3 CH 2 D in the ground states have been observed by a source-modulation spectrometer and analyzed to determine the two potential constants, V 3 and V 6 , simultaneously and also to assess the isotopic effects on the potential function. The results obtained for C 2 H 6 are V 3 = 2.882 ± 0.010 and V 6 = 0.020 ± 0.010 kcal/mole. The staggered conformation in ethane was established by observing microwave spectra of gauche CH 2 DCH 2 D. The r z structure of ethane was recalculated by adding precise rotational constants obtained in this work to previous microwave and infrared data.