Vibrational spectra of β-lactams—II. 1-Methyl-2-azetidinone and its deuterated compounds

The i.r. and Raman spectra of 1-methyl-2-azetidinone and its 3,3-d₂ and methyl-d₃ compounds have been recorded, and the observed bands have been assigned on the basis of the isotope effects and the normal coordinate analysis. Comparison of the CO and CN force constants between 2-azetidinone and 1-methyl-2-azetidinone indicates that, depending on amide resonance, these constants are related to each other. The solvent effects on the CO frequencies of four to seven-membered lactams have also been examined.     

Scaled quantum mechanical (SQM) force field and theoretical vibrational spectrum for benzonitrile

The complete harmonic force field of benzonitrile has been determined by ab initio Hartree—Fock calculations using a 4–21 Gaussian basis set. As force constants are systematically over-estimated at this level, the directly calculated force field was scaled by empirical factors previously optimized for benzene and HCN. Frequencies calculated from this scaled quantum mechanical (SQM) force field confirm the published experimental assignments for benzonitrile, benzonitrile-p-d and benzonitrile-d₅. Aside from the CH (and CD) stretching frequencies, which are strongly affected by anharmonicity, the mean deviation between the observed and calculated frequencies is below 9 cm⁻¹ for each isotopomer. Theoretical i.r. intensities reproduce the main features of the spectra semiquantitatively.     

Spectroscopic studies of lewis acid—base complexes: IV. An infrared study of oriented single crystals and normal coordinate analyses of trimethylamine—boron halide adducts

A total of 19 isotopic species of various (CH₃)₃NBX₃ complexes have been prepared and their IR spectra studied in the solid state in the region 40–4000 cm^?1. The isotopes involved are deuterium, boron-10, boron-11, and nitrogen-15 with X = F, CI, Br and I. C_3v symmetry has been preserved in all cases. Symmetry classifications of the fundamental molecular frequencies have been derived from polarized IR measurements on oriented single crystals in the range 250–4000 cm^?1. Errors and inconsistencies in previous studies have been resolved and revised vibrational assignments are proposed on the basis of the new data. Normal coordinate analyses have been carried out utilizing symmetry compliance constants without the assumption of point mass methyl groups. A common potential function containing 22 parameters for the A₁ class and 26 for the E class was utilized. The results support the proposed frequency assignments. B-N force constants obtained by inversion of the compliance matrices vary systematically from the chloro to the iodo complex. However, the B-N force constant for the fluoro complex falls between that for the chloro and bromo species; no systematic trend appears for the B-N compliance constants.     

A 19F NMR study of the transmission of electronic effects in triarylantimony and triarylbismuth compounds. A comparison with the nitrogen- and carbon-containing analogues

A ¹⁹F NMR study of the transmission of electronic effects has been made for the systems Ar₂EC₆H₄F-4 (E = Sb, Bi, CH, N). The fluorine chemical shifts obtained are correlated with the polar constants (Σσ^o and Σσ) of the substituents, suggesting that electronic effects are transmitted through the SbC_ar, BiC_ar and CC_ar bonds predominantly by an inductive mechanism, whereas the transmission through the NC_ar bonds is contributed to significantly by classical resonance effects due to competitive conjugation of the lone pair with the aromatic rings, and the substituents therein. A dual parameter correlation of the fluorine chemical shifts with the inductive (σ_I) and resonance (σ^o_R and σ_R) parameters of the substituents in the aromatic rings has led to similar conclusions. The inductive transmission through the bridging Sb and Bi atoms has been assigned to the absence of conjugation of lone pair and vacant d-orbitals of the metals with π-electron systems of the aromatic rings. On the basis of the values of the ? coefficients for the correlation equations obtained it has been established that the transmitting ability of the BiC_ar bonds is close to that of the C_alC_ar bonds and considerably lower than the transmitting ability of the NC_ar bonds.     

Vibronic spectra and normal coordinate analysis of substituted anilines—I. 2,3-, 2,4-, and 2,5-dimethylanilines

Vibronic spectra of 2,3-, 2,4-, and 2,5-dimethylanilines (DMA) have been recorded in the vapor phase and analysed primarily for ascertaining the frequencies of certain low lying fundamentals (< 1000 cm⁻¹) which were observed earlier in infrared and Raman spectra with rather poor intensities. In most cases, the investigation of vibronic spectra has supported the data observed in infrared and Raman spectra. However, a new frequency has been observed at ∼120 cm⁻¹ in each molecule and has been assigned to the CNH₂ out-of-plane wag. A detailed analysis of the vibronic spectra of these three molecules revealed that an interaction-induced blue-shift of nearly 1700 cm⁻¹ is caused in the (0, 0) band of all the three DMAs due to an electrostatic interaction between the NH₂ and CH₃ groups.The normal coordinate analyses (NCA) of the above three molecules have been carried out using a general valence force field (GVFF) with 21 principal and 30 interaction force constants, assuming an averaged structure, a planar geometry, and a C_s symmetry in each case. The fundamental frequencies were mostly taken from an earlier work reported from this laboratory. Some frequencies were also picked up from the investigation of the vibronic spectra reported herein. The force field calculations were carried out using the least-squares iterative technique. During final calculations, all the 51 force constants were allowed to iterate simultaneously, which reproduced the 52 experimentally observed fundamentals out of the 54 (35a′+ 19a″) expected ones within an average error of ± 1.0% with a reasonable potential energy distribution (PED) among the various normal modes.     

Resonance CARS spectra and vibrational analysis of fluorene-h10 and fluorene-d10 in T1 state

The observation of time resolved resonance CARS spectra of fluorene-h₁₀ and -d₁₀ in their T₁ states in room temperature solution are reported. Observed Raman frequencies are assigned on the basis of a crude normal coordinate calculation of the a₁ fundamentals and a CNDO/S-Cl calculation. The agreement between the calculated and observed values of ν₆ and ν₈ CC stretching frequencies confirms the orbital nature of the T₁ state.     

CN stretching and NO2 deformation vibrations and normal coordinate analysis of m-dinitrobenzene and m-dinitrobenzene-d4

Two bands appear for each CN stretching and nitro deformation vibration in the infrared and Raman spectra of m-dinitrobenzene and m-dinitrobenzene-d₄. The 907 cm^?1 bending mode in the vibrational spectra of m-dinitrobenzene undergo 30 cm^?1 upward shift upon d₄ substitution. A normal coordinate analysis pointed out that the 937 cm^?1 bending and 727 cm^?1 CN stretching vibrations as well as 18b CD in-plane deformation are mixed to a great extent. The other nitro bending mode undergo also an inverse isotopic effect (2 cm^?1 upward shift) due to coupling with the 18a CD in-plane deformation vibration.     

The microwave spectrum and structure of 1,1,1,3,3,3-hexafluoropropane,freon R236fa

The microwave spectra of three isotopic species of 1,1,1,3,3,3-hexafluoropropane (R236fa) (CF₃CH₂CF₃, ¹³CF₃CH₂CF₃ and CF₃¹³CH₂CF₃) were observed in the region from 4.5 to 18 GHz using a molecular beam Fourier transform microwave spectrometer (MB-FTMW). The rotational and centrifugal distortion constants were determined. The r_s (C–C) bond length and bond angle (C–C–C) of the molecule were determined to be 1.56(5) and 109(3) Å, respectively. The ab initio calculation of R236fa was executed at the MP2/6-31G(d,p) level. Comparison of the bond length and bond angle of R236fa with those in the other fluoropropanes revealed trends for the C–C bond length and CCC angle, depending on the number of fluorine atoms attached to the central carbon. A similar idea had been noted by Mack et al. from the electron diffraction studies of fluoropropanes.     

Emission spectra of supersonically cooled methyldiacetylene cations

The ā²E → X?²E (Σ= + $\text{1}\text{2}$, - $\text{1}\text{2}$) electronic transitions of rotationally/vibrationally cooled CH₃CCCCH^- cation, as well as the d₁-/d₃-/d₄-substituted species, were studied by emission spectroscopy. Ion emission was obtained by electron impact on the neutral species seeded in a helium supersonic free jet. Vibrational frequencies in both electronic states are inferred to within ±1 cm^-1. Spin-orbit splittings are observed and interpreted on the basis of non-linear vibronic couplings. Rotational subbands are observed, yielding rotational and Coriolis parameters as well as rotational temperatures.     

Vibrational spectra and normal coordinate calculations for trimethylselenonium-d0 and -d9 ions and dimethyl selenide-d0, -d6

The i.r. (4000-30 cm⁻¹) and Raman (4000-0 cm⁻¹) spectra of trimethylselenonium iodide and dimethyl selenide have been obtained, together with those of their deuterated analogues. All the active fundamentals of the trimethylselenonium ion have been assigned, assuming C_3ν molecular symmetry. Normal coordinate calculations have been carried out for the trimethyl-selenonium ion and dimethyl selenide and the SeC force constants of both the compounds are compared, including those of other corresponding chalcogenides.     

Substituent effect on the IR and PMR spectra of diphenylamines

PMR chemical shifts of various m- and p-substituted diphenylamines were correlated with σ-constants. However, the NH stretching frequencies of these amines were not linear with the σ- constants, and the irregularity in the ν_NHvs σ plot was interpreted by taking into accounts the following factors: the changes in hybridization of the imino N atoms, the normal electronic effect of the substituent and the steric effect on the planarity of the amine molecules. Results on the CN stretching absorptions were also presented.     

The vibrational spectrum of tetrafluoropropyne

The i.r. spectrum of gaseous tetrafluoropropyne has been measured from 4000 to 100 cm⁻¹, and all of the observed bands have been assigned. The e mode frequencies of the CF₃ group are similar to those of other CF₃CCX species, and even though the a₁ modes are less regular, the variations can be explained without changes in force constants other than those involving the CX bond. Several bands, particularly ν₁ and combinations with ν₁, show pronounced sequence structure due to excited levels of ν₁₀, the CCC skeletal bend.     

A DFT analysis of the vibrational spectra of nitrobenzene

Raman and FTIR, spectra of nitrobenzene, nb, and its isotopomers, nb-¹⁵N, nb-¹³C₆ and nb-d₅, were obtained and the fundamental vibrational modes assigned with the aid of a B3LYP/6-311+G** calculation, without the need for scaling of the force constants. The changes in vibrational coupling between the nitro and benzene groups upon certain isotopic substitutions are well modelled by the calculation, which is able to reproduce the isotopic shifts in frequencies for the nitro vibrations, as well as changes in IR intensities.     

The syntheses and coordination properties of M(CO)3X(DAB) (M = Mn,Re; X = Cl,Br, I; DAB = 1,4-diazabutadiene)

M(CO)₅X (M = Mn, Re; X = Cl, Br, I) reacts with DAB (1,4-diazabutadiene = R₁N=C(R₂)C(R₂)′=NR′₁) to give M(CO)₃X(DAB). The ¹H, ¹³C NMR and IR spectra indicate that the facial isomer is formed exclusively. A comparison of the ¹³C NMR spectra of M(CO)₃X(DAB) (M = Mn, Re; X = Cl, Br, I; DAB = glyoxalbis-t-butylimine, glyoxyalbisisopropylimine) and the related M(CO)₄DAB complexes (M = Cr, Mo, W) with Fe(CO)₃DAB complexes shows that the charge density on the ligands is comparable in both types of d⁶ metal complexes but is slightly different in the Fe-d⁸ complexes. The effect of the DAB substituents on the carbonyl stretching frequencies is in agreement with the A′(cis) > A″ (cis) > A′(trans) band ordering.Mn(CO)₃Cl(t-BuNCHCHNt-Bu) reacts with AgBF₄ under a CO atmosphere yielding [Mn(CO)₄(t-BuNCHCHN-t-Bu)]BF₄. The cationic complex is isoelectronic with M(CO)₄(t-BuNCHCHNt-Bu) (M = Cr, Mo, W).     

The thermodynamic effect of fluorine as a substituent: Allylic CF3 and CF2H

Cope rearrangements of 7,7,7-trifluoro-1,5-heptadiene and 7,7-difluoro-1,5-heptadiene were examined to gain quantitative understanding of the thermodynamic effect of allylic fluorine substitution. Group value contributions to ΔH_f's were thus able to be determined for allylic fluorine-substituted carbon groups: C(F)₃(C_d) = ?166.0, C(F)₂(H)(C_d)= ?107.6 and C(F)(H)₂(C_d) = ?52.2 kcal/mole.     

Infrared and Raman spectra and normal coordinate analysis of trans-1,2-dicyanocyclopropane and its deuterated isotopomers

Infrared and Raman spectra are presented for trans-1,2-dicyanocyclopropane and its 1,2-d₂- and 3,3-d₂-isotopomers. Most of the d₀ assignments agree with previous assignments of Strumpf and Dunker encompassing 26 of the 27 fundamentals. The 1,2-d₂ assignments are similar, but for the 3,3-d₂ species only 11 bands could be assigned. A complete ab initio quantum mechanical force field has been calculated for this molecule at the 6-31G* basis set level. This force-field was scaled and least-squares optimized using eight parameters for functionally related diagonal force constants and their geometric mean for off-diagonals. Both theoretical minimum energy and “experimentally corrected” geometries were used with no significant difference in results. An earlier calculation with a 4-31G basis set gave a similar frequency fit but different scaling factors. As an alternative approach, the scaled ab initio force field, was also used as a starting point for a more conventional refinement of 27 force constants which had a significant influence on the potential energy distribution. The remainder were fixed at the scaled ab initio values. This empirical force field resulted in a fit to 65 fundamentals of d₀, 1,2-d₂ and 3,3-d₂ to < 1% average error.     

Infrared study of some specificities of the dπpπ interaction in organogermanium compounds

The analysis is presented for the frequencies of stretching modes ν(GeH) in the IR spectra of organogermanium compounds R₂XGeH, RX₂GeH, RXYGeH, X₂YGeH and XYGeH₂ (where R is a substituent which does not make a d_πp_π bond with germanium, and X and Y are groups capable of d_πp_π interaction with germanium). It is shown that ν(GeH) in these compounds is dependent on both the I effect of R, X and Y, and the d_πp_π interaction in GeX and GeY bonds. If only one substituent capable of d_πp_π interaction with germanium is present, the value of such an effect is determined by itsσo_Rconstant. However, when germanium is bound to several substituents capable of d_πp_π interaction its magnitude depends on the effective charge at germanium which is determined by the inductive and mesomeric effects of X and Y. The data obtained are compared to the dependences observed in the IR spectra of similar organosilicon compounds.     

High resolution infrared analyses of fundamentals and overtones in isotopic ketenes

Rovibrational structure in the ν₅, ν₆, ν₇, ν₈ fundamentals and 2ν₅, 2ν₆ overtones in each of H₂¹²C¹²CO, H₂¹²C¹³CO, H₂¹³C¹²CO has been assigned and analysed. The effects of Fermi resonance between ν₇ and ν₂+ν₈ are corrected for, and a set of accurate unperturbed vibrational frequencies and ¹³C isotopic frequency shifts therefrom is obtained. The changing effects of the major Coriolis interactions involving the out-of-plane fundamentals, ν₅ and ν₆, 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 2ν₅ and 2ν₆ give evidence of major Fermi resonance interactions between both overtones and the ν₄ fundamental, which they straddle.