Harmonic Frequencies and Potential Energy Distributions of Partially Deuterated Methyl Cyanide

Although the vibrational spectra and force constants of CH₃CN and CD₃CN have been thoroughly studied, partially deuterated methyl cyanide has received much less attention. The infrared spectrum of CD₂HCN has only recently been reported1 and that of CH2DCN has not yet appeared. Normal coordinate analysis for neither partially deuterated species has appeared. We report here harmonic frequencies and potential energy distributions for both partially deuterated methyl cyanide species, CH₂DCN and CD₂HCN, based on force fields and structural parameters from CH₃CN and CD₃CN. The calculated frequencies for CD₂HCN are compared with the observed infrared frequencies. The vibrational interaction of the relatively high CN stretching frequency and the CD stretching frequencies is also discussed.     

The harmonic force fields of methylene chloride and dichlorosilane from combined microwave and infrared data

The harmonic force fields of methylene chloride and dichlorosilane have been obtained by combining the vibrational wavenumbers and centrifugal distortion constants of several isotopic species. Although enough data were available from earlier work for dichlorosilane, it was first necessary for methylene chloride to determine its distortion constants from microwave spectra. Transitions were measured up to J = 80 and J = 90 for CH₂Cl₂ and CD₂Cl₂, respectively, and the analysis gave accurate rotational constants, and quartic and sextic distortion constants. Ground-state effective, substitution, ground-state average and approximate equilibrium structures have been obtained for both molecules.     

Anharmonic force field for methanol

An ab initio quartic anharmonic force field for methanol has been calculated at the equilibrium position using the CCSD(T) method for the structure and the harmonic potential energy surface, and the MP4(SDQ) method for the anharmonic part of the surface. A triple zeta basis set was employed with symmetrized curvilinear internal valence coordinates in all calculations. The internal coordinate force field constants have been transformed into force constants in the dimensionless normal coordinate representation for various isotopomers. Vibrational term values for CH₃OH, CH₃OD, CD₃OH, and CD₃OD have been obtained using second order perturbation theory. Particular care has been devoted to the inclusion of Fermi resonance interactions between different vibrational states. A good accuracy has been achieved in the calculation of the fundamentals for all the isotopomers, the mean absolute error being 5.8 cm^?1.     

Infrared Spectrum and Vibrational Analysis of Bromochloromethane

Abstract

The i,r. spectrum of CH₂BrCl between 4000 and 200 cm^?1 has been investigated in the liquid phase. All the absorptions, except few features, have been identified and more than 50 bands including fundamentals, overtone and combination bands have been assigned. Anharmonicity constants from the characterized vibrational levels have been also determined. Data from liquid phase spectra are needed to correctly interpret the rather complex vapor phase features whose investigation is in progress now.     

The assignment of Co-C bond stretching vibrational frequency of CH3Co(DH)2H2O in IR and Raman spectra

Abstract

In order to aid assignment of Co-C bond stretching vibrational frequency of CH₃Co(DH)₂H₂O (DH=dimethylgIyoximato monanion) in IR and Raman spectra, its isotopic substitution CD₃Co(DH)₂H₂O has been synthesized and normal coordinate analyses on the two complex have been made. The bands were assigned in terms of potential energy distribution. The results provide definitive band assignment of the Co-C bond and Co-N bond stretching modes which are coupling at 511 cm^?1.     

An Improved Anharmonic Force Field of Difluoromethanimine, F2C NH

The anharmonic force field of difluoromethanimine, F₂C NH, has been reinvestigated theoretically using a coupled-cluster singles and doubles approach, augmented for structural optimization and harmonic force field by a contribution of connected triple excitations, CCSD(T). The cubic and quartic force constants have been obtained by numerical derivatives computed from analytical quadratic force constants calculated by second-order Møller-Plesset perturbation theory, MP2. The quadratic force constants and the equilibrium structure of F₂C NH have then been scaled by a global least-squares fitting procedure to the spectroscopic data and parameters experimentally determined for this molecule. This force field, obtained in the internal coordinates space and therefore valid for all isotopomers of difluoromethanimine, yields a complete set of spectroscopic molecular constants providing a critical assessment of the experimental rotational and centrifugal distortion constants, fundamentals, overtones, and combination bands determined so far for F₂C NH. In addition, the final force field can be used to make predictions of all important vibrational and rotational parameters which should be accurate and useful for new spectroscopic investigations.     

Methylene chloride: The mid-infrared spectrum of an almost vibrationally unperturbed molecule

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 vibrational spectra of some isotopic species of propionitrile

The infrared and Raman spectra of CH₃CH₂CN, CH₃CD₂CN, and CD₃CH₂CN, and the infrared spectrum of CH₃CH₂¹³CN were investigated in detail between 6000 and 100 cm⁻¹. Some infrared measurements of other isotopic species are also reported and partial assignments given. All fundamentals of propionitrile-d₀, -d₂, -d₃, and -¹³CN were assigned, together with a large number of mainly binary combination bands for which a general method of assignment is given. Several Fermi resonances were detected and the unperturbed positions of some of the levels involved were calculated. Special attention was paid to the CH stretching vibrations for which persisting wrong assignments exist in the literature, and to the methyl torsion frequencies which were determined for the four isotopic species above. A valence force field was calculated, and the potential energy distribution of the normal vibrations is tabulated.     

Fundamental vibrational frequencies and spectroscopic constants for the methylperoxyl radical,CH3O2, and related isotopologues 13CH3OO,CH3 18O18O,and CD3OO

Accurate spectroscopic and geometric constants for CH₃O₂, and its isotopologues ¹³CH₃OO, CH₃ ¹⁸O¹⁸O and CD₃OO, are predicted. Employing coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)], we obtain optimized equilibrium geometries using Dunning's cc-pVTZ basis set. A Taylor expansion of the potential energy surface, including all third-order and semidiagonal fourth-order terms in a basis of normal coordinates, yields anharmonic vibrational frequencies and vibrationally-averaged properties including the effects of anharmonicity. We detail the strong influence of Fermi resonances on the problematic ν₆ vibrational mode of CD₃OO, arriving at a value of 993?cm^?1; two previous experimental measurements of this mode appear to have been incorrectly assigned. Our computed energies for the low intensity ν₁₁ transition are in excellent agreement with experimental measurements performed for CH₃ ¹⁸O¹⁸O and CD₃OO, inspiring confidence that our results will serve as a guide for experimental measurement of this yet-unobserved quantity for the CH₃OO and ¹³CH₃OO isotopologues. Given the reliability of our force field, and considering the results of other experiments, we make a number of reassignments to previously recorded spectra, which eliminate large disagreements between experimental observations. The vibrational averaging of the rotational constants and geometries are also discussed for each isotopologue.     

Microwave spectrum of methylene fluoride-d2, CD2F2 in the excited vibrational states

The rotational spectra of ¹²CD₂F₂ in the ν₂, ν₃, ν₄, 2ν₄, ν₅, ν₇, ν₈, and ν₉ states were observed and assigned. Weak Coriolis interactions between ν₃ and ν₇, ν₃ and ν₉, and ν₅ and ν₇ were analyzed using approximate expressions for the rotational energy levels. The resonance between the ν₂ and the ν₈ state was found much stronger, and an effective two-dimensional Hamiltonian with the Coriolis term in the off-diagonal block was set up to analyze the spectra. The effect of the Fermi resonance between ν₃ and 2ν₄ was found to be very small.The ground-state spectrum of ¹³CD₂F₂ was observed and the rotational constants and the centrifugal distortion constants were determined. The data on ¹²CD₂F₂ and ¹²CDHF₂ were also improved very much in accuracy.The Coriolis coupling constants and the differences between two vibrational levels in resonance, which were determined by the analysis of the satellite spectra, are in good agreement with those obtained from vibrational spectra, except for the ν₂ band center, which is revised to 1170.3 cm^?1. The force constants were also checked using the centrifugal distortion constants of ¹²CD₂F₂, ¹³CD₂F₂, and ¹²CHDF₂.     

Vibrational analysis of ethylamines: Trans and gauche forms

Starting from force constant values calculated by an ab initio MO method ($\text{4-31G(N}{}^1\text{)}$), and by adjusting the diagonal elements, a practical force constant matrix (F) has been reached which could explain the observed infrared and Raman spectra (in the frequency range lower than 2000 cm^?1) of the gauche form of the ethylamine CH₃CH₂NH₂ molecule and five isotopic species CH₃¹³CH₂NH₂, CH₃CH₂¹⁵NH₂, CH₃CD₂NH₂, CH₃CH₂ND₂, and CD₃CD₂NH₂. The F matrix for the trans form of ethylamine was constructed by transferring ab initio $\text{4-31G(N}{}^1\text{)}$ values and by revising diagonal elements with conversion factors whose values are equal to the corresponding values of gauche form. A nearly complete set of assignments was achieved of the vibrational bands of ethylamines, observed so far in the spectral range 2000–100 cm^?1. In matrix isolation spectroscopy, two bands assignable to the NH₂ wagging vibrations of gauche and trans forms have been found at 775 and 782 cm^?1, respectively, for CH₃CH₂NH₂. They are at 768 and 774 cm^?1, respectively, for CD₃CD₂NH₂. From the intensity changes of these bands observed on changing the nozzle temperature in the matrix formation, the energy difference ΔE (gauche-trans) of these two conformers has been estimated to be 100 ± 10 cm^?1.     

The reaction of F atoms with CO in an argon matrix. Vibrational and electronic spectra of FCO

Upon codeposition at 14 K of an Ar:CO sample with an Ar:NF₃ sample that had been passed through a low-power microwave discharge in an Al₂O₃ tube, very prominent infrared absorptions of FCO and F₂CO were observed. Overtone bands of FCO were identified at 2032 and 3690 cm⁻¹, and their assignment was confirmed by both carbon-13 and oxygen-18 isotopic substitution. Further data were also obtained on the vibrational fundamentals of isotopically substituted F₂CO. A six-constant valence force potential has been derived for FCO using a least-squares force constant adjustment to the isotopic data and assuming a recently calculated structure. The photodissociation of FCO into F + CO was observed with a threshold near 2800 Å. The assignment of two overlapping electronic transitions of FCO between 3400 and 2300 Å is discussed.     

Infrared and Raman spectra and vibrational assignments of diiodomethane and its deuterated analogs

The infrared (from 4000 to 100 cm^?1) and Raman spectra of CH₂I₂ and CD₂I₂ have been recorded in the liquid and gaseous phases. Assignments have been made for all observed bands and, in the case of CH₂I₂, compared with those previously reported. Some bands appearing in the CD₂I₂ spectrum have been attributed to the presence of CHDI₂. The wavenumbers of the fundamental bands of CHDI₂ have been calculated from those of CH₂I₂ and CD₂I₂ using Brodersen and Langseth's rule, and compared with those observed in the CD₂I₂ spectrum.     

Microwave spectrum of methylene fluoride in excited vibrational states

The rotational spectra of ¹²CH₂F₂ in seven of the nine fundamental vibrational states and also in overtone and combination states involving the ν₄ mode were observed and assigned. Coriolis interactions between ν₃ and ν₇, ν₂ and ν₈, ν₃ and ν₉, and ν₅ and ν₇ were analyzed by using approximate expressions for the rotational levels. An effective Hamiltonian with the Coriolis term in the off-diagonal block was applied to stronger interaction between ν₃ and ν₉. Fermi resonance between ν₃ and 2ν₄ was found to be negligible. The ground state spectra of ¹²CH₂F₂ and of ¹³CH₂F₂ were remeasured to improve the accuracy of the rotational and centrifugal distortion constants. The Coriolis coupling constants and the energy differences between two vibrational levels in resonance, which were obtained through an analysis of the satellite spectra, are compared with the results derived from a normal coordinate analysis.     

Force field calculations of acetonitrile using CNDO/force method

The redundancy-free internal valence force field (RFIVFF) of acetonitrile is reported using CNDO/force method. The initial force field is set up by taking the interaction and bending force constants from CNDO force field and transferring stretching force constants from the force fields of chemically related molecules. The final force field is obtained by refining the initial force field using vibrational harmonic frequencies of CH₃CN,¹³CH₃CN, CH₃ ¹³CN, CH₃C¹⁵N, CD₃CN and CD₃ ¹³CN. The final force field thus obtained is found to be excellent on the basis of frequency fit and potential energy distribution.     

Centrifugal distortion and internal rotation analysis of the rotational spectra of N-methylmethanimine d0 and d5

The ground state millimeter-wave spectra of CH₃NCH₂ and CD₃NCD₂ have been measured. The rotational constants, centrifugal distortion constants, and barrier hindering internal rotation of the methyl group have been determined for both species. For the parent species I_α and ?(i,a) were also obtained, and for the perdeuteriated species the quadrupole coupling constants of ¹⁴N were determined.     

Infrared spectra of matrix isolated BH3NH3, BD3ND3, and BH3ND3

The infrared spectra of ammonia-borane, BH₃NH₃, and two of its deuterated isotropic species, BD₃ND₃ and BH₃ND₃, isolated in argon matrix at liquid hydrogen temperature have been measured. Well resolved bands for these three isotopic species have been observed for all the fundamentals. A complete frequency assignment based on C₃v molecular symmetry has been made. A set of force constants have been calculated from the data for the two isotopes BH₃NH₃ and BD₃ND₃ using a valence force field. The agreement between experiment and frequencies calculated from these force constants for the mixed isotopic species, BH₃ND₃, substantiates the present assignment.     

The methyl bromide molecule: A critical consideration of perturbations in spectra

This work gives an extensive critique of studies on methyl bromide and all its isotopic varieties with special stress on their rotational, vibrational, and rovibrational spectra. The rotational constants of more than 40 vibrational states of CH₃Br and 20 of CD₃Br, as well as of the ground states of all varieties, were critically examined and corrected where needed. An almost complete set of harmonic and anharmonic constants for CH₃Br was derived. From the set of rotation-vibration interaction constants, new accurate equilibrium constants A_e and B_e have been evaluated for CH₃⁷⁹Br, CH₃⁸¹Br, CD₃⁷⁹Br, CD₃⁸¹Br, from which the following equilibrium structure is obtained: r_e(C---H) = 1.0823 Å; r_e(C---Br) = 1.9340 Å; α(HCH) = 111.157°.     

High resolution Fourier transform infrared spectroscopy on CH2DI and CHD2I: evaluation of the ground state constants and analyses of the C-I stretching bands ν3

Abstract

The high resolution (0.0010cm^?1) Fourier transform infrared spectra of the partially deuterated methyl iodide molecules CH₂DI and CHD₂1 have been recorded and analysed in the ν₃ band regions around 510cm^?1. The fundamental band ν₃ is associated with the stretching of the C-I bond and the spectra appear therefore as an asymmetric rotor hybrid a/b-type band and hybrid a/c-type band for CH₂DI and CHD₂I, respectively. About 4700 transitions in the case of CH₂DI and about 3900 transitions in the case of CHD₂I have been assigned. The ground state rotational constants of CH₂DI and CHD₂I have been obtained using the ground state combination differences calculated from the assigned ν₃ transitions and 16 microwave transitions from literature. The S reduced Watson's Hamiltonian has been used in the calculations. In addition, the upper state parameters describing the v₃=1 vibrational states of these molecules have been determined. The obtained ground state constants as well as the upper state parameters have been compared to the corresponding constants of the symmetric top species CH₃I and CD₃I