Marlin D. Harmony

Molecular structures of gas‐phase polyatomic molecules determined by spectroscopic methods

Spectroscopic data related to the structures of polyatomic molecules in the gas phase have been reviewed, critically evaluated, and compiled. All reported bond distances and angles have been classified as equilibrium (re), average (rz), substitution (rs), or effective (ro) parameters, and have been given a quality rating which is a measure of the parameter uncertainty. The surveyed literature includes work from all of the areas of gas‐phase spectroscopy from which precise quantitative structural information can be derived. Introductory material includes definitions of the various types of parameters and a description of the evaluation procedure.

Microwave Spectrum and Structure of Bicyclo[1.1.0]butane

The microwave spectra of four isotopic species of bicyclo[1.1.0] butane have been investigated and the rotational constants of each species have been determined. Combining these data with those of the previously studied normal isotopic species, a complete set of structural parameters have been found. These include C1–C3 = 1.497 A, C3–C4 = 1.498 A, C3–C6 = 1.071 A, C–H (methylene) = 1.093 A, ∠C1C3H6 = 128°22′, dihedral angle, α, between cyclopropane rings = 121°40′, ∠HCH = 115°34′. These results are discussed with regard to the electronic structure of the molecule.

The equilibrium carbon–carbon single‐bond length in ethane

The recently developed scaled moment‐of‐inertia procedure for obtaining near‐equilibrium bond distances has been applied to ethane. The resulting prototype carbon–carbon equilibrium bond length is found to be 1.522±0.002 A, in good agreement with an earlier estimate based on electron diffraction results.

Laser-excitation spectrum of gas-phase CCl2

Abstract The gas-phase laser-induced fluorescence—excitation spectrum of dichlorocarbene (CCl 2 ) has been observed in the 498–534 nm spectral region. Sufficient resolution has been obtained to partially resolve the band structure arising from the symmetrical stretching and bending vibrations, which leads to vibrational parameters for these modes.

Microwave Spectrum, Structure, and Dipole Moment of Bicyclo[2.1.0]pentane

The microwave spectra of the normal and three singly substituted 13C isotopic species of bicyclo[2.1.0]pentane have been assigned and analyzed. Using the moments of inertia derived from these spectra the following structural parameters have been obtained for the carbon skeleton: C1 – C2=C3 – C4=1.528 A, C2 – C3=1.565 A, C1 – C4=1.536 A, C1 – C5=C4 – C5=1.507 A, and the dihedral angle α=67.26°. The disagreement of these results with those of an earlier electron diffraction study has been discussed. It seems likely that a reanalysis of the electron diffraction data will lead to concurrence with the results of the present study. We have compared our structural parameters to those of other cyclic systems and have discussed the essential constancy of the average C – C distances in cyclobutyl systems and also in cyclopropyl systems. Finally, Stark effect measurements have yielded a total dipole moment of 0.27 D.

The use of scaled moments of inertia for structural calculations

Abstract A procedure for obtaining molecular structural parameters from microwave spectral data is described. The method uses the same set of experimental ground state moments of inertia used for a substitution structure determination, but is based upon a least-squares fit of moments which are obtained by scaling the experimental I 0 values. The scaling is performed in such a way that the resulting moments of inertia, I m γ are comparable to Watsons I m moments and are thus close to the equilibrium moments. Initial tests and evaluation suggest that the method may lead to structural parameters which are better approximations to the r e structure than those obtained by the conventional r 0 or r s methods.

Microwave Spectra of Nitrogen‐Containing Molecules. IV. Conformation, Dipole Moment, and Quadrupole Coupling Constants of Cyclopropylamine

The microwave spectra of three isotopic species of cyclopropylamine have been investigated. The A, B, and C rotational constants in megahertz for C3H5NH2 are, respectively, 16269.95, 6723.00, and 5795.33; for C3H5NHD they are 15957.12, 6382.01, and 5504.06; and for C3H5ND2 they are 15592.62, 6091.39, and 5246.56. Quadrupole coupling constants have been measured and have the values χaa = 2.288, χbb = 1.841, and χcc = −4.129 MHz for the normal isotopic species. The dipole‐moment components are found to be μa = 0.43 D and μc = 1.11 D which lead to a total moment of 1.19 D. The molecular structure of cyclopropylamine has been discussed, and in addition to other structural parameters, we have found C–C = 1.520, C–N = 1.428, and amino H···H = 1.631 A.

The molecular structure of norbornene as determined by electron diffraction and microwave spectroscopy

Abstract The molecular structure of norbornene has been investigated in the gas phase by combining electron diffraction data with microwave spectroscopic rotational constants. The interatomic distances (rg) and bond angles were obtained by applying a least squares program to the refined experimental molecular diffraction intensities. The CC bond length was found to be 1.336 ± 0.002 A while the ) bond length was 1. 529 ± 0.007 A. Other bond lengths and angles included (IUPAC numbering system was used for norbornene): C1-C6 = 1.550 ± 0.020 A, C1-C7 = 1.566± 0.005 A, C5-C6 = 1.556 ± 0.005 A, C-Have. = 1.103 ± 0.003 A, ∠C1C2C4 = 95.3°. The dihedral angle between planes C1C2C3C4 and C1C6C5C4 is 110.8 ± 1.5° while that between C1C2C3C4 and C1C7C4 is 122.3°. The moments of inertia calculated from ED structure are in good agreement with microwave spectroscopic values.

The use of scaled moments of inertia in experimental structure determinations: extension to simple molecules containing hydrogen

Abstract The scaled moment of inertia structure theory described previously has been extended to treat simple molecules containing hydrogen atoms. With the aid of model force-field computations for HCN and HNC, simple methods have been developed to permit corrections for the large zeropoint vibration-rotation effects occurring upon deuterium substitution. The methods utilize a limited amount of ground state moment of inertia data and yield near-equilibrium XY distances (0.001 A or better) for HCO+, HCN, HN2+, and HNC. A near-equilibrium structure for HOC+ is reported for the first time: r (HO) = 0.975 ± 0.002 A and r (OC) = 1.1570 ± 0.0005 A .

Structural determinations using scaled moments of inertia: Further tests with heavy-atom systems

Abstract The scaled moment of inertia theory described previously has been used to compute experimental near-equilibrium structures (rmϱ) for several heavy-atom triatomic molecules including SO2, S2O, and ClNO. The results in all cases are in generally better agreement with the known re structures than are the conventional rs structures. Computations of the vibration-rotation parameters involved in the theory, using anharmonic model force fields, have aided understanding of the sucess of the method. They have shown also that reliable structures can be expected when the “parent” molecule is either an all heavy-isotope isotopomer or an all light-isotope isotopomer. In conjunction with earlier work, the present results suggest that rmϱ structures should provide reliable alternatives or substitutes for rm structures with no increase in data requirements.