Victor W. Laurie

Anharmonic Potential Constants and Their Dependence upon Bond Length

Empirical study of cubic and quartic vibrational force constants for diatomic molecules shows them to be approximately exponential functions of internuclear distance. A family of curves is obtained, determined by the location of the bonded atoms in rows of the periodic table. Displacements between successive curves correspond closely to those in Badgers rule for quadratic force constants (for which the parameters are redetermined to accord with all data now available). Constants for excited electronic and ionic states appear on practically the same curves as those for the ground states. Predictions based on the diatomic correlations agree with the available cubic constants for bond stretching in polyatomic molecules, regardless of the type of bonding involved. Some implications of these regularities are discussed.

Influence of Vibrations on Molecular Structure Determinations. II. Average Structures Derived from Spectroscopic Data

Formulas are given which enable structural parameters for the average molecular configuration in the ground vibrational state to be calculated for some simple types of molecules. The data required are the observed effective moments of inertia and harmonic force constants. No knowledge of anharmonic constants is necessary. The average structural parameters have a well‐defined physical meaning and are directly comparable with diffraction results. Polyatomic molecules for which explicit calculations are given are CO2, CS2, H2O, SO2, O3, NO2, CH4, HCN, and C2H2. It is found that the average bond lengths involving H are usually 0.003–0.005 A longer than the corresponding D bond. For bonds involving heavier elements isotopic differences are smaller but nonetheless significant. Implications of the results for the general problem of structural determination are discussed.

Influence of Vibrations on Molecular Structure Determinations. III. Inertial Defects

The relationships between various types of moments of inertia and vibration—rotation corrections are developed from general theory, with emphasis on the qualitative physical features and the quantities which may be calculated without knowledge of anharmonic force constants. The inertial defects of planar triatomic and tetratomic molecules and nonplanar molecules with a plane of symmetry are considered, and it is found that simple approximations, which depend mainly on the one or two modes of lowest frequency, give results within 10%—20% of the experimental values. The application of inertial defect corrections in structure analysis is illustrated for the calculation of HH distances in CH2Cl2, SiH2F2, and CH3CXO molecules (X = H, F, Cl, Br).

Microwave Spectrum of Dimethylamine

Microwave spectra of 12 isotopic species of dimethylamine have been assigned for the ground vibrational state. Splittings arising from the inversion of the amino hydrogen have been observed and an inversion barrier of 4.4 kcal/mole has been estimated. From the rotational constants the structural parameters have been calculated and the conformation of the methyl groups has been determined. The structural parameters obtained are: r(CN) = 1.466 A, r(NH) = 1.022 A, r(CH) (average) = 1.09 A, ∠CNC = 112.2°, ∠HCH (average) = 108.6°, ∠HNC = 108.8°. Evidence for an asymmetric methyl group is discussed. The 14N nuclear quadrupole coupling constants in the principal quadrupole axis system are χxx = 3.04 MHz, χyy = 2.01 MHz, and χzz = −5.05 MHz, where the x axis is normal to the symmetry plane. Analysis of the Stark effect gave a dipole moment of 1.01 D.

Deuterium Isotope Effects on Molecular Dipole Moments by Microwave Spectroscopy

A special parallel‐plate microwave spectrometer has been constructed and the effect of deuteration on the dipole moment in the ground vibrational state of seven different molecules has been measured. The molecules studied were: methylacetylene, propane, methyl fluoride, fluoroform, vinylidene fluoride, methylsilane, and methylgermane. With the exception of fluoroform, where an effect of 0.001 D was observed, the magnitude of the isotope effect was found to be of the order of 0.01 D. A simplified interpretation of the isotope effects leads to information about the signs of the dipole moments in these molecules.

Microwave Spectrum, Ring‐Puckering Potential Function, Ring Structure, and Dipole Moment of Cyclobutanone

The microwave spectrum of cyclobutanone in the ground state and the first 10 excited states of the ring‐puckering vibration has been assigned. Most features of the observed variation of rotational constants with ring‐puckering state and of the far‐infrared spectrum are accounted for with the ring‐puckering potential function V(Z) = ν0[12Z2 + 0.4918Z4 + 1.598 exp(−1.0Z2)], with ν0 = 29.85 cm−1. Z is a dimensionless coordinate for the ring‐puckering motion. A barrier of 7.6 ± 2 cm− exists at the planar ring conformation; with the υ = 0 level lying 9.2 cm−1 above the top of the barrier. A perturbation of the υ = 8 state is discussed. The ground‐state rotational spectra of the three 13C monosubstituted species have also been assigned and the structure of the ring determined. With the carbonyl carbon labeled C1 and other carbons numbered sequentially around the ring, the ring structural parameters are r(C1C2) = 1.527 + 0.003 A, r(C2C3) = 1.556 ± 0.001 A, ∠C2C1C2′ = 93.1 ± 0.3°, ∠C1C2C3 = 88.0 ± 0.3°, ∠C2C3C2′ ...

Microwave Spectra and Structures of Difluoroethylenes

The microwave spectra of the C12 and C13 species of cis‐difluoroethylene‐d1 and cis‐difluoroethylene‐d2 have been investigated in the region of 8–36 kMc. Effective rotational constants for the ground vibrational state have been determined and are combined with previous microwave data to obtain the structural parameters: rCC=1.324 A, rCH=1.089 A, rCF=1.335 A, ∠FCC=122.1°, ∠HCC=124.0°.The microwave spectrum of vinylidene fluoride has been reinvestigated and the rotational constants for C13 species obtained. These data lead to a structure: rCC=1.315 A, rCH=1.079 A, rCF=1.323 A, ∠FCF=109.1°, ∠HCH=121.8°.A comparison of the structures with data for vinyl fluoride and ethylene shows that fluorine substitution produces a systematic shortening of both the CC and CF bonds.

Structure of Cyclopentadiene

The microwave spectra of the three monosubstituted 13C species of cyclopentadiene have been investigated. The effective rotational constants of these species together with those of the main isotopic species give the structural parameters for the ring. Labeling the methylene carbon as C1 and numbering the other carbons sequentially we obtain: r(C1C2)=1.509 A, r(C2C3)=1.342 A, r(C3C4)=1.469 A, ∠C1C2C3=109.3°, ∠C2C3C4=109.4°, ∠C2C1C5=102.8°.

Structure of Isobutylene

The microwave spectrum of (CH3)2C=C13H2, CH3C13H3C=CH2, and (CH3)2C=CHD has been studied in the region 8–40 kMc/sec. The effective rotational constants of these species combined with previous microwave data give the structural parameters: rCC(single)=1.507 A, rCC(double)=1.330 A, average rCH(methyl)=1.08 A, rCH(ethylenic)=1.088 A, average <HCH (methyl)=107°, <HCH (ethylenic)=118.5°, <CCC=115.3°.

Microwave Spectrum, Structure, and Dipole Moment of Carbonyl Fluoride

The microwave spectrum of carbonyl fluoride, COF2 has been investigated in the region 8–30 kMc. The following effective rotational constants have been obtained (Mc): C12O16F219C13O16F219C12O18F219a11813.4511814.6611813.48b11752.9911747.2710878.54c5880.915879.815653.32 The structure calculated from these constants is rCF = 1.312 A, rCO = 1.174 A, ≰FCF = 108.0°. From measurements of the Stark effect the dipole moment is found to be 0.951±0.010 D.