B. P. Dailey

Determination of Electronic Structure of Molecules from Nuclear Quadrupole Effects

Nuclear quadrupole coupling constants in molecules depend on the nuclear quadrupole moments and the variation in electrostatic field at the nucleus. It is shown that this variation of electric field is usually simply related to the molecular electronic structure, being primarily dependent on the way in which valence electrons fill the lowest‐energy p‐type orbits. Structural information which can consequently be obtained from known quadrupole coupling constants is discussed. Hybridization of the normal covalent bonds of N, Cl, and As with at least 15 percent s character is clearly shown. The alkali halides appear to be almost purely ionic; the quadrupole coupling data allow no more than 3 percent covalent character. In addition to molecular structure, some nuclear quadrupole moments are approximately evaluated by use of the theory developed here.

The Ionic Character of Diatomic Molecules

A relation between the electronegativity difference of two bonded atoms and the ionic character of the bond is obtained for singly bonded diatomic molecules in the gaseous state. The relation is based primarily on the wide variety of nuclear quadrupole coupling constants recently measured by microwave techniques. However, dipole moments of diatomic molecules are also used and discussed. Both quadrupole coupling constants and dipole moments indicate strongly that bonds involving electronegativity differences greater than 2 are almost completely ionic. Certain effects of hybridization, overlap, and polarization on considerations of ionicity are discussed.

Proton Chemical Shifts for the Alkyl Derivatives

The chemical shifts of a series of methyl, ethyl, propyl, and isopropyl derivatives have been studied in an effort to determine the distance and angular dependence of any contribution to the chemical shift arising from magnetically anisotropic substituent groups. It was found that electron withdrawal effects play a dominant role in determining the chemical shifts for methyl derivatives. The nearly exactly linear relation of the methyl shifts to electronegativity for the methyl halides seems to rule out any large influence due to magnetic anisotropy. The ethyl shifts were found to be determined by electron withdrawal effects plus a factor which acted equally at the α and β positions. It was found that this factor arises from the carbon‐carbon bond. By assigning a chemical shift due to the presence of the C–C bond, which is dependent in size on the substituent attached to the α carbon, the apparently anomalous frequencies observed for the alkyl derivatives can be accounted for. A possible explanation for th...

Nuclear Quadrupole Effects and Electronic Structure of Molecules in the Solid State

Measured nuclear quadrupole effects in solids are discussed, compared with nuclear quadrupole coupling in gases, and correlated to some extent with molecular structure in the solid state. The iodine crystal affords a good example of intermolecular interactions in the solid state. In this crystal, nuclear quadrupole effects combined with crystallographic information show two intermolecular covalent bonds per atom, each of about 9 percent importance.

NMR Determination of Some Deuterium Quadrupole Coupling Constants in Nematic Solutions

The analysis of the NMR spectra of suitable nematic liquid crystal solutions appears to offer a convenient and desirable method for the determination of small nuclear quadrupole coupling constants for molecules in the liquid state. Nuclear quadrupole coupling constants have been determined for deuterium in the CD3 groups of methyl iodide, methyl bromide, and acetonitrile and for deuterium in fully deuterated benzene. Values of e2qQ / h measured along the molecular symmetry axis were found to be 66.9 ± 1.2 kHz for CD3I, 63.0 ± 1.0 for CD3Br, and 54.5 ± 1.5 for CD3CN. If the assumption is made that the C–D bond has rough axial symmetry then values of e2qQ / h along the bond axis are found to be 180 ± 5 kHz for CD3I, 171 ± 4 for CD3Br, and 165 ± 5 for CD3CN. e2qQ / h along the bond axis was found to have a value of 194 ± 4 for C6D6.

Microwave Spectra of Some Isotopically Substituted Phenols

The rotational spectra of C6H518OH, C6D5OH, and C6D5OD were successfully analyzed to obtain their respective rotational constants in the region from 20 000 to 31 000 Mc/sec. The splitting of the rotational transitions due to the hindered internal rotation of the hydroxyl group was noted in C6H518OH and C6D5OH while the spectrum of the C6D5OD isotope was unsplit. As is the case in C6H516OH, the high barrier to internal rotation produced a doublet spectrum, where the splittings were effectively independent of the J transition and correspond to 115 and 103 Mc/sec for the C6D5OH and C6H518OH isotopes, respectively. The experimental rotational constants are: IsotopeA(Mc/sec)B(Mc/sec)C(Mc/sec)C6D516OH4682.677±0.0062422.815±0.0041596.930±0.004C6D516OD4654.378±0.0052342.102±0.0041558.383±0.004C6H518OH5650.076±0.0082487.321±0.0051727.232±0.005.It was found that the structure of the C–O–H fragment could not be uniquely determined from the rotational constants. Two structures were found which were essentially the sa...

Microwave Spectrum of Bromocyclobutane

Rotational transitions of four isotopic species of bromocyclobutane have been observed. For C4H7Br79, [Complex chemical formula] or [Complex chemical formula] the rotational constants are A = 10 003.4±13 Mc, B = 1629.41±0.03 Mc, C = 1488.48±0.03 Mc; for C4H7Br81, A = 10 002.6±13 Mc, B = 1615.14±0.03 Mc, and C = 1476.50±0.03 Mc. The values for the α‐deuterated compound are, for C3H6CDBr79, A = 9534.7±13 Mc, B = 1613.67±0.03 Mc, and C = 1486.24±0.03 Mc; for C3H6CDBr81, A = 9533.4±13 Mc, B = 1599.55±0.03 Mc, and C = 1474.24±0.03 Mc. A set of structural parameters which reproduce these constants within 1.8 Mc were obtained with an electronic computer: bond distances Cα–Cβ = 1.540±0.003 A, Cβ–Cγ = 1.548±0.003 A, C–Br = 1.939 A, C–H = 1.096 A, and C—D = 1.087 A; bond angles CβCγCβ = 88° 06′±08′, CβCαCβ = 88°41′±08′, HCγH = 110°44′, HCβH = 108°44′, HCαBr = 111°, angle of CβCαCβ plane with C–Br = 131°00′±08′, and dihedral angle = 29°22′±08′ (the dihedral angle is the angle made by the normals of the CβCαCβ and Cβ...

Proton Chemical Shifts in Polysubstituted Benzenes

The ring proton chemical shifts of an extensive series of polysubstituted benzenes may be represented, with an accuracy of about 0.1 part per million, as simple sums of substituent shielding constants, except in the case of ring protons next to substituents which are ortho to one another.The small, uniform deviations from additivity of the 46 shifts measured in a series of para disubstituted benzenes are well represented by a linear variation, so that the chemical shift of proton 2 in a 1,4‐substituted benzene may be expressed as δ2=d0(R1)+γ(R1)dm(R4). The polarizability γ decreases as d0 increases.The values of the shielding parameters d are consistent with a mechanism dominated by the pi‐electron charge distribution, although other effects are present, especially at the meta position, which cannot be accounted for in a consistent manner. It can be shown that the molecular‐orbital theory of substituent interaction with the pi‐electron system can yield a variation of polarizability with charge density of ...

Proton NMR Spectra of Disubstituted Benzenes

Precise values of the coupling constants and chemical shifts at infinite dilution in cyclohexane have been determined for an extensive series of related disubstituted benzenes. A method of rigorous analysis of the A2B2 proton spectra of the para‐disubstituted benzenes is presented.

Microwave Spectrum and Structure of Formic Acid

The microwave spectra of five isotopic species of formic acid have been investigated, and values of the rotational constants B and C have been obtained. For HC12OOH, B=12 055.1 Mc, C=10 416.0 Mc; for HC12OOD, B=11 762.5 Mc, C=9970.1 Mc; for DC12OOH, B=12 055.6 Mc, C=9955.8 Mc; for DC12OOD, B=11 759.9 Mc, C=9534.1 Mc; and for HC13OOH, B=12 053.7 Mc, C=10 378.9 Mc. The following structural parameters have been fitted to these rotational constants: rC–H=1.085 A, rC=O=1.245 A, rC–O=1.312 A, rO–H=0.95 A, ∠OCO=124°18′, ∠COH=107°48′. The C=O bond has been estimated to have 75% double‐bond character. The torsional frequency was found to be 667±41 cm—1 and a barrier height of roughly 17 kcal/mole is indicated.