Leland C. Allen

Electronic Structure and Inversion Barrier of Ammonia

Ab initio molecular orbital wavefunctions have been constructed for the planar and pyramidal conformations of ammonia in its ground electronic state. These solutions are very close to the Hartree–Fock limit and exhibit a lower total energy (ETequil = − 56.22191 hartree) than any other such calculations which have been carried out. The computed inversion barrier is 5.08 kcal/mole (exptl = 5.8 kcal/mole) and results almost solely from the differential mixing of d‐type polarization functions in the planar and pyramidal geometries. In contrast to the conclusions of much earlier work, the inversion barrier may be quantitatively obtained within the framework of the molecular orbital (Hartree–Fock) approximation. Other properties of ammonia that have been determined and discussed are: binding energy, heat of formation, correlation energy, dipole moment, population and energy component analysis, force constants, and the Walsh diagram.

Theory of the Hydrogen Bond: Electronic Structure and Properties of the Water Dimer

High‐accuracy molecular‐orbital calculations using essentially Hartree–Fock quality atomic orbitals as a basis have been carried out on different geometries of the water dimer. Different basis sets have been considered. The molecular‐orbital approach is shown to well represent the geometry and heat of formation (− 5.3 kcal/mole) of the water dimer as well as general infrared spectral properties of the hydrogen bond. The individual molecular‐orbital energies are shown to increase for the electron acceptor and to decrease for the electron donor. This trend in energies is proposed as a quantitative organizing principle for not only H‐bond formation but all donor–acceptor interactions.

Origin of Rotational Barriers. I. Many‐Electron Molecular Orbital Wavefunctions for Ethane, Methyl Alcohol, and Hydrogen Peroxide

An investigation into the origin of rotation barriers has been carried out with the aid of LC(Hartree—Fock) AO MO SCF calculations on CH3CH3, CH3OH, and H2O2. The general shape and magnitudes of the barriers are reasonably well represented by the wavefunctions and one may conclude that the origin of rotational barriers is to be found within the framework of the Hartree—Fock approximation. It is also found that study of several molecules, particularly ones of low symmetry, is required for a detailed understanding of barriers.The electronic energy has been decomposed into one‐ and two‐electron parts. The relative phase and relative amplitude of these two components provide the best invariants available for analyzing the barrier mechanism. On the other hand the nuclear—nuclear repulsion is shown not to be an appropriate invariant. A physical picture of the barrier mechanism constructed from the electronic‐energy decomposition depicts the barrier origin as a balance between the interactions of lone pairs, bon...

Proton and carbon-13 chemical shifts: Comparison between theory and experiment

Abstract A series of ab initio 1 H and 13 C NMR chemical shifts are presented for all molecules for which gas-phase experimental measurements exist. Quantitative agreement with this large set of data is achieved by the use of gauge-invariant atomic orbitals in an SCF perturbation theory approach. The effect of basis set completeness on these 1 H and 13 C chemical shifts is also examined. The 4-31G basis set is found to provide internally consistent results and give satisfactory agreement with gas-phase experimental data. Errors within 6% for 1 H shifts and 3% for 13 C shifts result. Increasing the basis set to the 6-31G * level does not significantly improve the agreement. For 1 H shifts only, the 3-21G basis set is adequate. The validity of the particular computational approach employed here is further substantiated by comparison to another ab initio magnetic shielding method.

Theory of the Hydrogen Bond: Ab Initio Calculations on Hydrogen Fluoride Dimer and the Mixed Water–Hydrogen Fluoride Dimer

High‐accuracy molecular orbital calculations have been carried out on different geometries of the hydrogen fluoride dimer and the mixed water–hydrogen fluoride dimer. A zigzag (near linear) structure is predicted for the hydrogen fluoride dimer with a dimerization energy in reasonable agreement with experiment. One geometry of the mixed water–hydrogen fluoride dimer has a very large stabilization energy (10 kcal/mole), and a microwave experiment is proposed to determine its exact structure. Changes in molecular properties and charge distribution upon dimer formation are calculated and a dimer rotational barrier determined.

Geometry of Molecules. I. Wavefunctions for Some Six‐ and Eight‐Electron Polyhydrides

Potential surfaces have been investigated for BH2+, BH2−, NH2+, BeH2, BH3, CH3+, and BeH3− by means of LC (Hartree—Fock) AO MO SCF wavefunctions. One‐electron‐energy‐versus‐angle diagrams were determined for these AH2 and AH3 molecules, and comparison with the widely used empirical diagrams of Walsh demonstrates a quantitative similarity between them. In addition, mathematical inequalities are derived relating changes in the sum of one‐electron energies with angle to changes in the total energy with angle. Aided by these expressions, we are able to make direct connection between Walshs qualitative rules and the physically well‐defined ab initio solutions.

Energy component analysis of rotational barriers

Abstract The energy components, V att  V ne and V rep  T ÷ V nn ÷ V ee are employed in the analysis of the barrier mechanism. Molecules treated are: CH 3 CH 3 , CH 3 NH 2 , CH 3 OH, CH 3 CH 2 F, CH 3 CHO, CH 3 CHCH 2 , H 3 BNH 3 , NH 2 NH 2 , NH 2 OH, HOOH, HONO, and HOCHO.

Internal Rotation Barriers for Hydrazine and Hydroxylamine from Ab Initio LCAO—MO—SCF Wavefunctions

LCAO—MO—SCF ab initio wavefunctions with atomic basis orbitals of double‐zeta accuracy have been constructed for N2H4 and NH2OH and the potential‐energy curves versus internal‐rotation angle have been determined. Similar wavefunctions for CH3CH3, CH3OH, CH3NH2, and H2O2, reported previously, showed a quantitatively useful correlation with experimental barrier heights and shapes. Since experimental information on the N2H4 and NH2OH barriers is incomplete or unknown, the theoretical results presented here may help guide experimental work. Decomposition of the total energy into one‐ and two‐electron components has been found previously to help elucidate the physical origin of the rotational barrier, and results for the species studied here are given. The wavefunctions presented here (and those reported previously for the species noted above) are the most accurate available for molecules possessing a rotational barrier about a single bond, and comparison is made with the results of the other existing ab initi...

Strong, positive‐ion hydrogen bonds: The binary complexes formed from NH3, OH2, FH, PH3, SH2, and ClH

A systematic, a b i n i t i oelectronic structure analysis of the strong, positive‐ion hydrogen bond is reported. Energies and wave functions have been obtained at the 4‐31G level for the twenty‐one complexes, (B⋅⋅⋅H–A)+, where A,B=NH3, OH2, FH, PH3, SH2, ClH. The A–H bond length (r 1), the B⋅⋅⋅A separation (R(, and the angle (ϑ) measured relative to the symmetry axis of B have been optimized. Calculated dimerization energies E D are found to be in reasonable agreement with experiment. Charge density difference plots of these complexes exhibit a remarkable similarity to the pattern of alternating charge gain and loss known for the neutral H‐bonded dimers. The proton donor is characterized by a charge gain region between A and the proton and charge loss on the proton; the electron donor by a charge loss between B and the proton. Four of the complexes (HF⋅⋅⋅H⋅⋅⋅FH)+, (H2O⋅⋅⋅H⋅⋅⋅OH2)+, (HCl⋅⋅⋅H⋅⋅⋅ClH)+, (HCl⋅⋅⋅H⋅⋅⋅FH)+, have unusually short internuclear separations and show large charge gain around the protons. This is the first theoretical evidence of a transition from predominantly electrostatic to predominantly covalent binding in hydrogen bonding and it corroborates a recent experimental X–N study. An estimate of the amount of charge lost from the proton, Δq H, has been obtained from the difference plots and is found to bear a linear relation with the dimerization energy E D for a series of complexes with a single proton donor. The inverse relation between E D and the difference in monomerproton affinities, ΔPA, reported in the literature for substituted pyridinium ions, is shown to hold as well for all A and B. Our calculated results also give a quantitative demonstration of the recently proposed inverse relationship between proton position and ΔPA. Several useful new organizing principles have been found: (a) R varies linearly with r 2, the B⋅⋅⋅H distance. Covalent bonding in the four complexes noted above is indicated by deviation from this line. Crystal structures taken from the literature also obey this relationship and the satisfactory agreement between experiment and calculations show that the 4‐31G basis is adequate for predicting strong H‐bond geometries. (b) R N /R?1.2, where R N is the A⋅⋅⋅B separation in neutral hydrogen bonds. (c) E D displays a smooth inverse relation to r 2 for a sequence of complexes with a single electron donor.

Extended Hückel Theory and the Shape of Molecules

Ab initio MO SCF wavefunctions and one‐electron energies are compared with extended‐Huckel‐theory values for the polyatomic molecules BeH2, BH2+, BH2−, NH2+, H2O, BeH3−, BH3, CH3+, Li2O, F2O, LiOH, and FOH. A remarkable similarity in the nodal structure, order of the energy levels, prediction of molecular shape, and changes in hybridization and ionization potential with angle is observed. The origin of this correspondence for molecular shape is found to be a general theorem relating changes in the sum of one‐electron energies to the total energy and implying a low information content to the angular dependence of molecular energetics. Analysis of the Huckel molecular orbitals also shows that quantitative evaluation of many other chemically interesting quantities is not to be expected. These investigations help give a quantum‐mechanical underpinning to extended Huckel theory, elucidate its a priori capabilities, and lead to proposals for realizing more quantitative predictions from this model.