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The consequences for lambs of infection over the winter with Teladorsagia circumcincta were quantified by deliberate, trickle infection of selected animals at 7 months of age. Infected and control uninfected animals were each allocated into four groups, relatively resistant animals on a normal diet, relatively resistant animals on an isocaloric diet supplemented with urea, and relatively susceptible animals on the same two diets. Resistance and susceptibility was assessed by faecal egg counts following natural infection during the summer preceding the deliberate infection. During the deliberate infection egg counts remained low and most parasites recovered at necropsy were inhibited larvae. Nonetheless, infection reduced weight gain, decreased albumin and fructosamine concentrations and provoked a noticeable pepsinogen and eosinophil response. As most larvae were inhibited these responses may have been largely a consequence of immuno-inflammatory responses in the host rather than the direct action of parasites themselves. Relatively resistant animals on the supplemented diet allowed fewer larvae to establish and had higher fructosamine concentrations, higher albumin concentrations and decreased pepsinogen responses. Therefore, a combination of relatively resistant sheep and nutritional supplementation appears most efficient at controlling infection.

Vibrational assignments for CH 2 D 2 are made from a detailed study of its gas-phase infrared spectrum below 17 000 cm −1 , up to six quanta of CH stretching excitation. Some 77 overtone and combination vibration levels are identified. These allow extraction of empirical values for approximately one-half of the 45 normal mode anharmonicity constants, and their comparison with recent ab initio predictions. Severe Fermi resonance perturbations at low excitation levels in both CD 2 stretching manifolds preclude the direct extraction of any of the 17 associated anharmonicity constants. However, observation of seven Fermi resonance diads below 4000 cm −1 permits the accurate definition of the corresponding cubic resonance parameters, and the determination of unperturbed CD 2 stretching fundamental wavenumbers. A simple local mode interpretation of the unperturbed stretching vibrational manifold in CH 2 D 2 is attempted. This then yields the 10 corresponding normal mode CH and CD stretching anharmonicity constants consistent with the model applied.

The number of accumulated spectroscopic data sensitive to the harmonic force constants of methyl silane has been augmented recently through high resolution infrared gas-phase studies of 12 CH 3 SiH 3 , 13 CH 3 SiH 3 and 12 CH 3 SiD 3 species (J.L. Duncan, A.M. Ferguson and D.C. McKean, J. Mol. Spectrosc, 168 (1994) 522). This permits the determination of an empirical harmonic force field, in which 24 A, and E species force constants out of the total of 36 are independently defined in terms of 72 data across five isotopic species. The empirical constants are compared with those from a scaled ab initio force field using a basis set of double zeta plus polarization quality, where very good agreement is found throughout. Ultimately, a best estimate general harmonic force field is presented in which the 12 undermined and numerically small interaction force constants are constrained to their ab initio predicted scaled values.

The adult ES products of Dictyocaulus viviparus are a source of protective antigens against challenge in the guinea pig laboratory model. High levels of acetylcholinesterase (AChE) activity are present in these products and these enzymes are immunogenic in infected cattle. Here, the potential role of these enzymes in protective immunity was investigated using a fraction enriched for AChE to immunise guinea pigs. The antibody response stimulated by immunisation with AChE-enriched ES products and the worm burdens obtained following challenge with infective larvae were compared with those in animals immunised with whole ES products and challenge controls. The AChE-enriched preparation stimulated high levels of enzyme-specific antibody in immunised animals, which was not the case for those which received unfractionated ES products. Worm burdens of guinea pigs which received the AChE-enriched fraction were significantly lower than those obtained in adjuvant controls. The animals which received the unfractionated ES products were not significantly protected against challenge. These results suggest that AChEs may be potential candidates for incorporation in a sub-unit vaccine against D. viviparus .

The ν5 + ν9 A-type infrared combination band of C2D4 at 4627 cm-1 has the superficial appearance of an unperturbed absorption. Assignment, however, reveals the effects of Coriolis interaction with an inactive vibration level, ν2 + ν8 + ν9, which lies within 0·4 cm-1 of ν5 + ν9. Through the K dependence of the interaction, the K a = 0 subband J structure of ν5 + ν9 is completely unperturbed, while higher K a subband J structures assigned are displaced varyingly up to 3·5 cm-1. Due to almost exact degeneracy of interacting rovibration levels, for the K a = 3–7 subbands, two series of J structures are identified in the spectrum, and two series of subbands are identified in the overall Q branch for K a = 1–8, the ‘inactive’ transitions stealing intensity from those of ν5 + ν9 through the Coriolis interaction. Analysis of the assignments made yield unperturbed upper state parameters and the magnitude of the Coriolis interaction parameter.

Somatic extracts and excretory/secretory (ES) products of the adult stage of the cattle nematode, Dictyocaulus viviparus, were examined for acetylcholinesterase (AChE) activity. Both were found to contain activity which had an optimum pH of 9.5, however, the adult ES products contained over 200 times more AChE activity per unit protein. Gel electrophoresis and specific enzyme staining revealed 5 migratory isoforms of AChE which were common to adult ES products and adult homogenates. Comparison of L3 with L4 and adult extracts indicated that the AChE were only produced by later developmental stages of this parasite. The antigenicity of D. viviparus AChE was demonstrated by binding to serum IgG from naturally and experimentally infected calves but the enzymes were not recognized by calves vaccinated twice with 400 Gy-irradiated larvae. This is the first report of helminth AChE release by a parasitic nematode in a pulmonary location. The presence of these enzymes in such high amounts in the ES products, along with their immunogenicity, suggests that they might have an important role to play in the immunobiology of D. viviparus in the lungs.

Second-order perturbation theory corrections to effective Fermi resonance coupling constants have been derived analytically within the framework of the traditional rectilinear normal coordinate approach to the analysis of anharmonic molecular vibrations. These corrections have been evaluated for a range of methyl halide molecules (including deuterated isotopomers) and are found to rationalise qualitatively discrepancies between the empirical effective Fermi resonance parameters and theoretical normal coordinate cubic force constants.

The IR spectra of C2H3D and C2HD3 have been studied up to 3 quanta of excitation of CH and CD stretching (

Previous studies of the CH (and CD) stretching vibration manifolds of the series of molecules H 2 CCH 2 , H 2 CCD 2 , H 2 CCCl 2 , and H 2 CCF 2 have been extended to cover all spectroscopic observations made to date on excited state vibration levels. From these, sets of normal mode anharmonicity constants have been assembled over the four molecules. These are individually incomplete, but exhibit a considerable degree of transferability of constants which relate to the normal vibrations associated with the moiety. In turn, this allows the vibrations of the group to be characterized in terms of a set of 24 averaged normal mode anharmonicity constants, which reproduce 77 spectroscopic observations for ethylene-H 4 up to 16 500 cm −1 with an rms error of 4.30 cm −1 .

The A 1 and E species fundamentals of 12 CH 3 SiH 3 , 13 CH 3 SiH 3 , and 12 CH 3 SiD 3 samples have been analyzed by gas-phase infrared spectroscopy at resolutions of up to 0.05 cm −1 . Precise vibrational and rovibrational parameters have been obtained for most fundamentals over all three isotopic species. The K structure of the E species fundamentals which was sufficiently resolved was readily assigned in most cases, rendering subsequent analyses straightforward. Analyses of the methyl group deformation fundamentals, ν 9 , previously misassigned to the more intense ν 11 + ν 12 combination band, are presented. Definitive vibrational band origins for the silyl group deformation fundamentals, ν 10 , are presented, partly from observations of the overtone ν 0 10 . Analyses of five perpendicular combination and overtone bands are also given. Evidence of localized perturbations are observed in most of the 37 bands studied.

The complete general harmonic force field of methyl flouride was recalculated using the most recent literature frequency, Coriolis ζ, and centrifugal distortion data for 12 CH 3 F, 13 CH 3 F, 12 CD 3 F, 12 CHD 2 F and 12 CH 2 DF. The anharmonic corrections applied to the observed frequency data and the adopted molecular geometry are considered to be more realistic than those used hitherto. There is excellent overall agreement between the fitted force constants and the highest quality ab initio force field currently available.

The CH stretching anharmonicity in CH 2 CF 2 and CH 2 CCl 2 has been investigated through application of the local mode model to vibration levels up to 14500 cm −1 . Good reproduction of observed data is achieved in each case, but whereas a “normal” anharmonicity constant of ≈−58 cm −1 is obtained for CH 2 CCl 2 , an anomalous value of ≈−52 cm −1 is determined for CH 2 CF 2 . By comparison of the spectra, the cause of the problem is traced to two critical misassignments due to the effects of Fermi resonances at different levels of excitation in CH 2 CF 2 . A “normal” anharmonicity constant is then obtained. The Fermi resonances cannot be quantified satisfactorily, due to lack of sufficient observations and their complexity. The local mode parameters determined for each molecule are supported by available structural information.

Previous research has indicated that supplementing an apparently adequate diet with additional protein improves both host resistance and resilience in lambs infected with Haemonchus contortus. The present study tested the influence of supplementation with non-protein nitrogen (urea). Helminth-naive Hampshire Down lambs were given an apparently adequate basal diet or a diet supplemented with urea. The lambs were then infected with Haemonchus contortus for 10 weeks. Supplementation with urea had no discernible effect on resistance to infection; faecal egg counts, worm burdens, worm lengths and mean number of eggs per adult female worm did not differ between the 2 groups. However, lambs on the supplemented diet showed better resilience; they had greater packed red cell volumes, higher plasma albumin concentrations and increased liveweight gain compared to lambs on the basal diet. The loss of appetite following infection was less in lambs fed the urea-supplemented diet. The observed effect of urea supplementation was seemingly due to greater food consumption as well as the better diet.

Four fundamental bands in the infrared spectrum of 1,1-ethylene-d2 have been studied at a resolution of ca. 0·05 cm-1: the two type-B bands v 6 and v 10 and the two type-C bands v 7 and v 8. Analysis of a set of 409 ground state combination differences enables a complete set of ground state rotational and quartic distortion constants to be determined. The effect of the a-axis Coriolis interactions between the four bands is analysed, and upper state vibrational and rotational parameters for all four bands are obtained. Two localized higher order Coriolis perturbations are identified and characterized, and the band centre of the inactive twisting vibration v 4 is estimated. From the overlapping and weak type-A band v 3, a few upper state parameters are determined.

A thorough investigation of the infrared spectra in the gas phase and polycrystalline films of normal and 10B enriched samples of B2H6 and B2D6 has been made with a resolution of ∼0.3 cm−1. The data from this study, in conjunction with those from previous Raman studies, enable a complete and unambiguous assignment of all 18 fundamental vibrations of B2H6 and B2D6 to be made. All previously uncertain or unassigned fundamentals are located by direct observation in the crystal phase and by means of one or more infrared or Raman active combination bands. Boron isotopic vibration frequency shifts are used to analyze a number of important Fermi resonances. Good product rule fits then result in all symmetry species. Assignments in the regions 1900-1800 and 1500-1400 cm−1 respectively of B2H6 and B2D6 are clarified. Force field calculations predict a value of |ζ9,14a| close to zero, which removes an earlier difficulty in the assignment of the ν9 infrared active rocking fundamental. The absence and presence, respectively, of a center of symmetry in the α and β crystal phases are demonstrated.

The availability of twelve precisely determined ground state spectroscopic rotational constants for isotopic diboranes and a physically realistic harmonic potential function has enabled the ground state r 0, substitution r s, zero-point average r z and equilibrium r e structure parameters to be calculated. Vibrational effects render the ground state and substitution structures of doubtful physical significance. Their removal leads to zero-point average and equilibrium structure parameters which are entirely compatible with, but more accurately determined than, those available from electron diffraction observations. The equilibrium structure is fully substantiated by parameters calculated from a recent extension to the Watson mass-dependence method.

The infrared spectrum of H2CCD2 was investigated in the region 2000-1200 cm−1 at a resolution of ∼0.05 cm−1. Complete analyses of four type-A bands are reported-the CC stretch, ν2, the CH2 deformation, ν12, and the overtones of the CD2 and CH2 wagging fundamentals, 2ν7 and 2ν8. Localized perturbations are identified and taken into account in the analyses, enabling the perturbing vibrational levels to be located accurately and the interaction parameters to be determined. In the case of ν12, Fermi resonance with 2ν10 has a global effect, and causes a bandhead to be formed in the Ka subband series.

The =CH2 and =CD2 stretching vibrational overtones of H2C=CD2 have been studied up to V= 6 and V= 3, respectively. We report their interpretation in terms of a transition from normal to local modes, involving Fermi resonance with the CC stretching and CH2 scissoring vibrations. We discuss the alternative representation of the vibrational Hamiltonian matrix in local mode and normal mode basis functions, and conclude that the normal mode basis offers greater flexibility in representing small anharmonic couplings with other modes.

The infrared gas phase spectra of 12CH2Cl2, 13CH2Cl2, and 12CD2Cl2 have been studied in the region below 6200 cm−1 under conditions of high resolution. Some 30 vibrational levels can be identified for each isotopic species and assigned unequivocally in terms of the band contours displayed. Direct observation has been made of the very weak ν2 fundamentals in all species, and of the “inactive” torsion fundamental of CD2Cl2. Rotational analyses have been performed on the observed Q-branch features of over 30 bands. For each isotopic species, it is found, with one exception, that all vibration levels fit accurately the simple second-order perturbation expression involving ν′s and x′s. The sole exception in each species is the overtone region of the CH2(CD2) stretching vibrations. Here anharmonicity effects bring vibrationally interacting levels into close enough proximity for resonance effects to become just slightly more than of second-order importance. Full analyses including Fermi resonance are made. The effects of the Darling-Dennison resonance between the overtones of the CH stretching fundamentals are observed and corrected for in terms of a simple assumption. Most of the resulting anharmonicity constants bear isotopic relationships similar to those established for H2O and D2O. It is concluded that, with the exception of the CH(CD) stretching overtone region, methylene chloride isotopomers behave as vibrationally unperturbed molecular systems in the mid-infrared region.

The use of recently determined highly precise inertial data for various isotopic modifications of methyl isocyanide has enabled the ground-state average, or r z , structure to be determined to within very narrow limits. Harmonic corrections to ground-state rotational constants have been calculated using a high-quality, experimentally determined harmonic force field. The derived zero-point inertial constants are sufficiently accurate to enable changes in the CH bond length and NCH bond angle on deuteration to be determined. The present r z structure determination is believed to be a physically realistic estimate of the ground-state average geometry of methyl isocyanide.

Rotational assignments in the ν6 + ν9 type-A band and the ν9, ν6 + ν11, and ν1 + ν6 type-B bands of ethylene-1,1-D2, recorded at a resolution of ∼0.03 cm−1, enable the ground state rotational constants to be determined much more accurately than previously. A significant change in the A0 constant is noted. All upper states suffer perturbations to their rotational structures. Analyses, excluding the areas of perturbation, still enable the excited state constants to be determined with considerable precision.

The infrared spectrum of CHF 2 Cl has been recorded between 15 000 and 350 cm −1 . The Fermi resonance between levels involving ν 4 and 2ν 6 is analysed in bands extending from 800 cm −1 to 7000 cm −1 leading to a best value of k 466 = ± 14.98 cm −1 . In conjunction with the recent results of Amrein, Dubal and Quack, Molec. Phys. 56,727 (1985); estimates are reported for 38 out of 45 possible x ij constants. A variation in the relative intensity of the two Q branches associated with ν 1 , on cooling the gas cell, indicates that a hot band contributes to the upper branch at 3024.55 cm −1 . However, other evidence suggests that the latter arises also from the combination ν 2 + ν 7 + ν 9 , in a very weak, close resonance with ν 1 at 3021.27 cm − . A number of anomalous band contours are reported.

It is demonstrated that Fermi resonance perturbations occur in the ν7 CH stretching fundamentals of CH3CD3 and C2H6. These render existing spectroscopic parameters completely spurious and account for the anomalous ζ constants found. A very simple model reproduces perfectly the high-resolution data in the literature with conventional parameters for a CH stretching vibration.

Ground state rotation and quartic distortion constants were obtained for 11B2D6 from the analysis of high resolution (∼0.05 cm−1) Fourier transform infrared spectra. The bands studied comprised the ν17, ν18 type A, and ν14, ν9 + ν15 type C bands of 11B2H6 and the ν16, ν17, ν18 type A, ν8 type B, and ν14 type C bands of 11B2D6. In the case of 11B2H6, the authors' ground state data were combined with those of Lafferty et al. obtained from a previous study (J. Mol. Spectrosc. 33, 345–367 (1970)) at comparable resolution of the ν16 type A and ν8 type B fundamentals. Information on the ground state rotational energy manifold of 11B2H6 was accumulated up to J = 23, Ka = 18, and of 11B2D6 up to J = 32, Ka = 22. This permitted rather precise determination of the distortion constants ΔJ0, ΔJK0, ΔK0, although δJ0 and δK0 proved to be too small (< 10−7 cm−1) and were constrained to values calculated from the force field. Sets of upper state parameters were determined for all vibrational levels studied. Although these appear to be essentially unperturbed globally, several localized perturbations were observed and identified.

Analysis of the rovibration spectrum of CHD 2 CN enables the rotation parameter ( A o —B o ) to be determined, from which the value of A o may be found using the microwave B o and C o constants. The use of A o (CHD 2 CN) in conjunction with all other available B o and C o constants for isotopes of methyl cyanide enables the ground state geometry to be defined within fairly narrow limits. The CH bond length is in agreement with the value predicted from the CH vibration frequency of CHD 2 CN, and with the value calculated from the results of a recent analysis of the infrared spectrum of CD 3 CN.

The general harmonic force field of methylene chloride has been calculated without the necessity of imposing constraints, through the use of a set of 74 observables over eight isotopic species. These include vibration frequencies, 37 Cl and 13 C isotopic frequency shifts, and quartic centrifugal distortion constants. All force constants are well defined by the data with one exception, and this takes a value close to that predicted by transfer from methyl chloride, although with an uncertainty larger than itself. The force field is used to predict precise isotopic distortion constants, in terms of differences from the accurate experimental values for CH 2 35 Cl 2 and CD 2 35 Cl 2 , and to calculate Coriolis interaction constants which will be of assistance in high-resolution rovibration studies. Alternative definitions which may be used for the deformation coordinates of an XY 2 Z 2 molecule are considered. That which associates each deformation uniquely with either the XY 2 or XZ 2 group is clearly preferred.

It is demonstrated that the use of heavy isotopic differences in quartic distortion constant data contributes to a significant improvement in the definition of the empirically determined harmonic potential function of a molecule, provided the data are determined with sufficient precision. Formaldehyde is used as a test case.

The effects of first order Coriolis interactions and vibrational l type resonances in the infrared spectrum of the band system 2ν9 ±2, ν9 ±1+ν10 ±1, 2ν10 ±2 of gaseous allene-d4 have been studied at a resolution near 0·05 cm-1. Sets of spectroscopic constants for each of the vibrations are determined, as well as the Coriolis interaction parameter which couples all three levels. The relative magnitudes and signs of the intensity parameters of the band system have been derived from contour simulations. A number of local perturbations are discussed qualitatively.

Samples of methyl acetylene with 13C isotopic substitution at all three C positions have been prepared and their IR spectra studied. The resulting vibration frequencies yield accurate frequency shift data, which are of great value in increasing the precision with which the harmonic potential function for the molecule is determined. A total of 23 of the 30 force constant parameters can be estimated. The remaining constants, which link basis symmetry coordinates separated by at least one bond or one non-common atom, are assumed to be zero. In the course of the spectroscopic study, a Fermi resonance between the CH stretch vibration v1, and (v3 + 2vO9) was discovered, which becomes of first-order importance in the 13C substituted species. The effects of this resonance have been elucidated.

Ground state rotational constants over 8 isotopic species have been used to determine precise structural parameters for methylene chloride, with the assistance of the recently determined empirical general harmonic force field. The isotopic zero-point inertial constants are used to estimate equilibrium structural parameters, through a recently developed mass-dependence method, which yields values of r e (H··H) = 1.7876 A and r e (Cl··Cl) = 2.9292 A.

An ab initio gradient calculation of the general harmonic force field (GHFF) of methylene chloride at the HF-SCF/6-31 G** level has been undertaken. The results compare favourably with those determined empirically, thus demonstrating a good predictive capability at this level of sophistication for a molecule possessing two third row halogen atoms. The ab initio results reveal small inadequacies in the empirical force constants relating to the CH2 moiety, which may be corrected by a careful re-appraisal of the accumulated spectroscopic data. The resulting empirical and scaled quantum mechanical (SQM) sets of force constants are then in very good accord. Predicted ab initio cubic force constants for methylene chloride are also calculated.

Studies of five comparatively unperturbed infrared active bands in the spectrum of 10B2D6 were undertaken with a resolution of ca. 0.05 cm−1. These comprise three type-A bands (ν17, ν18, and ν5 + ν15), one type-B band (ν8), and one type-C band (ν14). Ground-state rotational and quartic centrifugal distortion constants were determined for the first time from a total of over 400 combination differences. Sets of upper-state parameters were determined for all five bands studied, and the effects of a number of minor Coriolis interactions between fundamental vibrations are discussed.

A total of 114 spectroscopic data accumulated from 6 isotopic species of diborane have been used to permit a precise determination of the empirical harmonic potential function to be made for the first time. Of the 33 independent force constants, 30 can be determined with numerical significance. The physical significance of the values is probably best demonstrated by the rather close correspondence throughout with scaled ab initio force constants from two sources, only one rather large disagreement in magnitude being found. The potential function reproduces all known frequency, isotopic frequency shift, Coriolis ζ, and centrifugal distortion Δ data to high accuracy. It is used to calculate parameters which will be of value to further spectroscopic and structural studies, to enable accurate analyses to be made of a number of major Fermi resonance interactions present in the spectra, and to explain the anomalously small Coriolis interaction between the infrared active rocking and wagging fundamentals. The advantages of alternative symmetry coordinate definitions are considered, particularly in relation to making comparisons between diborane and ethylene.

High-resolution infrared studies of isotopic ethylenes below 2000 cm −1 have been commenced with a Nicolet FTIR spectrometer. Accurate vibration and rotation parameters for the v 12 fundamentals of C 2 H 4 and C 2 D 4 are determined from spectra recorded with 0.05 cm −1 resolution. Excellent band contour simulations confirm that these bands are unperturbed throughout their range.

The gas-phase i.r. spectrum of CH 3 Br has been studied up to 14 000 cm −1 . Some 32 new vibration levels are located accurately, involving up to 5 quanta ( V = 5) of excitation in CH stretching. Reproduction of a total of 72 vibration levels is achieved through a joint treatment of CH stretching vibrations in a local mode basis, other vibrations in a normal mode basis, and with the inclusion of two Fermi resonances. The local mode approach satisfactorily accounts for the effects of the large Darling—Dennison vibrational interactions which occur between CH stretching modes. Fermi resonances between CH stretching and overtones of both methyl group deformation vibrations (ν 2 and ν 5 ) are treated explicitly. Interacting levels become quasi-degenerate at V = 3 in the case of 2ν 5 , and at V = 5 in the case of 2ν 2 . The analysis permits the determination of a complete set of 27 physically representative anharmonicity constants for CH 3 Br, four of which are interrelated through the local mode model. Data reproduction between 600 and 14 000 cm −1 is achieved with an r.m.s. error of 2.55 cm −1 .

Existing spectroscopic data for allene-H4, -D4, and -1,1-D2 are improved and augmented by gas-phase Raman and solid-phase infrared studies. A number of 13C vibration frequencies are identified in natural abundance by each technique. A total of 58 input data enable 22 of the 23 force field parameters to be determined, the 13C frequency shift data removing the ambiguity of choice between two sets of A1 and B2 species force constants. The remaining interaction force constant is constrained to the predicted ab initio value of Botschwina and Pulay. The force field is used to determine the ground-state average (rz) structure of allene. Observed trends in the H-D isotopic effects on the rz structures of ethylene, allene, and ketene are in accordance with those expected.

Five bands in the infrared spectrum of ethylene-D4 have been studied with a resolution of 0·02 cm-1, the A-type ν11 fundamental and and ν1 + ν11 combination, and the B-type ν9 fundamental and ν5 + ν11, ν2 + ν9 combination bands. From a set of 635 combination differences the ground state rotation and distortion constants are determined. Assignments for transitions with J ⩽ 30, Ka ⩽ 20 for B-type bands and Ka ⩽ 14 for A-type bands enable upper state vibration frequencies and rotational constants to be determined for all bands studied. The effects of Coriolis interaction through ζ1,5 c are observed in the ν1 + ν11 and ν5 + ν11 bands, and a value of the Coriolis constant is determined. A number of other higher order resonances in upper vibrational states are identified and characterized.

The recently determined empirical/ab initio general harmonic force field of ketene is used to correct existing ground state rotational constants over five isotopic species of ketene to their zero-point average values. The latter determine structure parameters of high precision for this fundamentally important molecule. A marked asymmetrical change in the zero-point CH 2 group structure on partial deuteration is both predicted and found, such that the CH bond shortens and the CD bond lengthens, an effect contributed to by the significant amplitude of the very low frequency bending coordinates On complete deuteration, a small but “normal” zero-point CH bond shortening is determined and predicted. Equilibrium structure parameters are estimated to be r e (CH) = 1.0753(17) A, r e (CC) = 1.3142(5) A, r e (CO) = 1.1609(4) A, ∠ e (HCH) = 121.76(33)°.

The present spectroscopic structural information on diborane derives from infrared studies of B2H6 and B2D6 species only. Due to the impossibility of selective isotopic substitution in diborane, and the consequent coexistence of a number of isotopic species in any partially deuterated sample, the most probable source of further structural information of quality will be from microwave studies of asymmetrically deuterated species. To assist in the assignment of the overlapping spectra that will occur, accurate rotation and quartic distortion constants are presented for the ground states of all isotopic diboranes in terms of existing zero-point average structural parameters and isotopic changes in these calculated through the harmonic potential function. Sets of fundamental anharmonic vibration frequencies are calculated in order that interference from low-lying vibrations with significant populations at ambient temperatures may be anticipated.

Analysis of the rovibration spectrum of CHD2NC has enabled the rotation parameter (A0B0) to be determined. Knowledge of the microwave B0 and C0 constants enables A0(CHD2NC) to be determined, which, when used in conjunction with all other B0 and C0 constants for isotopic methyl isocyanides, allows the ground state structure to be defined within fairly narrow limits. The CH bond length is much shorter than that determined by the substitution technique (rs structure). It is now in almost perfect agreement with the value predicted by the CH vibration frequency in CHD2NC, and falls in the range of CH bond lengths exhibited by all other symmetric top methyl group molecules.

Rovibrational structure in the ν 5 , ν 6 , ν 7 , ν 8 fundamentals and 2ν 5 , 2ν 6 overtones in each of H 2 12 C 12 CO, H 2 12 C 13 CO, H 2 13 C 12 CO has been assigned and analysed. The effects of Fermi resonance between ν 7 and ν 2 +ν 8 are corrected for, and a set of accurate unperturbed vibrational frequencies and 13 C isotopic frequency shifts therefrom is obtained. The changing effects of the major Coriolis interactions involving the out-of-plane fundamentals, ν 5 and ν 6 , are manifest in varying isotopic displacements of equivalent Q branch structures between the isotopic species studied, from which accurate isotopic vibrational shifts are obtained. Variable anharmonicity constants associated with the vibration frequencies of 2ν 5 and 2ν 6 give evidence of major Fermi resonance interactions between both overtones and the ν 4 fundamental, which they straddle.

The infrared active v 7 CD wagging fundamental band of ethylene-D4, has been studied with a resolution of ∼0·05 cm-1. A full upper state rovibration analysis is achieved, once the various effects of the indirect Coriolis interaction with the inactive v 4 torsion vibration are taken into account. Several localized perturbations are observed in the spectrum, all of which are identified. A number of these are shown to derive from energy level crossings between v 4 and v 7. A simultaneous fit to these allows the frequency of the inactive v 4 vibration to be estimated accurately.

In the absence of a direct experimental determination of A 0 for a symmetric top methyl group molecule, the relations used to determine the off-axis hydrogen coordinates by the mono-substitution technique are shown to lead to the prediction of abnormally long CH bonds. The molecules methyl iodide and methane, for which A 0 constants for the CH 3 I and CH 3 D species have been determined directly, and for which the ground and equilibrium state geometries are well known, are used as test cases. Methyl chloride, methyl cyanide, and methyl acetylene, for which reliable ground state geometries are known, are also considered. In order to obtain the A 0 rotational constant for CH 2 DI required in this study, an analysis of the rovibration spectrum, of CH 2 DI was carried out. The results of this work are given in the Appendix.

Infrared gas phase spectra of CH 3 NC, 13 CH 3 NC, CH 3 15 NC, CH 3 N 13 C, CD 3 NC, 13 CD 3 NC, and CD 3 15 NC have been studied in detail, in order to determine accurate fundamental vibration frequency displacements on heavy isotopic substitution and Coriolis interaction constants for the degenerate fundamental vibrations. These data, along with the fundamental frequencies of CH 3 NC and CD 3 NC and centrifugal distortion constants for six isotopic species, form a self-consistent set of 105 data, which enable all 20 parameters of the general harmonic potential function of methyl isocyanide to be determined within narrow limits. Comparison with the predictions of ab initio calculations reveals disagreement in the values of three interaction constants, apparently well outside experimental error. Undetermined centrifugal distortion and Coriolis interaction constants are calculated for CH 3 NC and CD 3 NC to assist in future spectroscopic studies.

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 ν 7 C -type CH 2 rocking and ν 8 A -type CH 2 wagging fundamentals of CH 2 Cl 2 have been recorded at a resolution of ∼ 0.04 cm −1 , and the ν 2 + ν 8 A -type combination at a resolution of ∼ 0.005 cm −1 . All three bands are analysed in terms of the quartic asymmetric rotor model, yielding precise vibrational and rotational parameters. Although each band appears to suffer minimal perturbation effects, assignment proved difficult due mainly to the effects of the overlapping spectrum of CH 35 2 Cl 37 Cl, but also to the small magnitude of the B and C rotational constants (∼ 0.1 cm −1 ). In the case of ν 7 , band contour simulations were required in order to confirm or refute possible assignments. The data obtained for ν 8 and ν 2 + ν 8 enable parameters to be predicted for the extremely weak ν 2 CH 2 deformation fundamental, only recently observed in the gas phase spectrum.

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.

Normal coordinate calculations have been carried out for a number of molecules of three-fold symmetry containing the silyl group. All available data on frequencies and Coriolis interaction constants for all isotopic species (where possible) were used in force constant refinement calculations. Since these data were not sufficient to fix the general harmonic force field (GFF), the silyl group force field was constrained according to the model which has been successfully applied to the methyl group in previous papers in this series. In general, the refinements were made independently for each molecule, and the results show a pleasing degree of uniformity for the silyl group when the nature of the fourth atom attached to the Si atom is taken into account. On the basis of the transferability of force constants in almost identical molecules, some assignments for silyl acetylene and methyl silyl acetylene are questioned, and alternative assignments proposed.

The infra-red spectrum of di- t -butyl peroxide has been obtained in the gas, liquid and solid states between 1500 and 400cm −1 *. The Raman spectrum of the liquid has also been re-investigated and qualitative polarization data obtained. The two sets of t -butoxy frequencies appear to combine to give modes of A − species intense in the Raman effect and motions of A − ” species strongly active in the infra-red. OO stretching motion is associated with the pair of Raman frequencies 904 and 862cm −1 .