Molecular orbital theory of the electron structure of organic compounds. XXIV. Geometries and energies of small boron compounds. Comparisons with carbocations

Abstract

Ab initio molecular orbital theory is used to predict geometries, stabilities, and charge distributions of I7 neutral one-and two-heavy-atom molecules containing boron and C, N , 0, or F. At the STO-3G level, geometries and conformational preferences are found to correspond to those previously calculated for the isoelectronic carbocations. However, the boron atom is found to be a stronger u donor and weaker R acceptor than C + . At the 6-3 IC* level, boron is seen to be stabilized in the following ways: by attachment of R donors ( "2, O H , F), which stabilize BH2X relative to BH3 by 53-58 kcal/ mol; by hyperconjugation, which stabilizes H2BCH3 by 12 kcal/mol; by dimerization, 2BH3 -B2H6, predicted to be exothermic by 20 kcal/mol (6-31G*) or 37 kcal/mol if the previously determined correction for correlation energy is applied; or by complexation with Lewis bases, leading to H3BNH3 (stable relative to BH3 + NH3 by 21 kcal at 6-31G*) and H3BOH2 ( 6 kcal), but not H3BFH (-8 kcal). Triplet ground states are predicted for BH, BCH, and B2H2. H2BNH2 and H2BOH are predicted to be planar with barriers to nonrigid rotation of 29 and 14 kcal/mol, respectively, while H2BCH3 has a negligible barrier, and H2BBH2 prefers a perpendicular D2d conformation by I O kcal/mol, relative to planar. Stabilities deduced from stabilization energies, calculated heats of hydrogenation, heats of formation, and bond energies follow the order HzBBH2 < H2BCH3 < H2BNH2 < HzBOH < H2BF; B-H bonding in the latter three compounds (X = "2, O H , F) is nearly double in character. For unsaturated species, the stability order is HBBH < HBCHz < HBNH < HBO, where HB=NH dnd HB=O have nearly triple bonds. Polarization functions are seen to be important in describing the bonding in B2H6 and the H3BNH3 and H3BOH2 adducts.The study of boron is becoming an increasingly important area of chemical i n t e r e~t .~.~ Although a number of theoretical studies have appeared, the majority have dealt with boron hydrides3g and boron-nitrogen compounds;3h there has been no systematic study of small organic molecules containing boron and other first-row elements. In this paper, we examine a number of such compounds, focusing on those aspects of electronic and geometric structure most closely related to our previous studies of first-row organic system^.^-^ As many of the species considered here are unknown, emphasis will be made on predicting structures, stabilities, and relationships to known species, particularly the isoelectronic carbocations.This study has several purposes. First, an examination of coordinate, multicenter, and covalent bonding in these structures will suggest similarities and differences between compounds of boron and those of heavier first-row atoms, thus supplementing our understanding of the interactions occurring in small organic molecules. Second, we wish to gain a deeper understanding of carbocations by comparing them with this series of electron-deficient, but uncharged, molecules. Third, these calculations are the necessary first step toward pre...

Thermochemistry of Molecules in the B−N−Cl−H System: Ab Initio Predictions Using the BAC-MP4 Method

“…Gurvich et al recommend a heat of formation for H 3 B:NH 3 of -27.5 ( 3.6 kcal mol -1 , which is somewhat more negative than the BAC-MP4 prediction of -20.0 ( 2.0 kcal mol -1 . The BAC-MP4 prediction of 31.3 kcal mol -1 for the B-N bond energy is in reasonable agreement with other predictions: 34.7 kcal mol -1 (MP4(SDTQ)/6-311G**), 38 30.7 kcal mol -1 (MP2/ TZ2P), 1 27.6 kcal mol -1 (CEPA-2), 66 27 kcal mol -1 (CEPA), 37 and 21 kcal mol -1 (6-31G*) 40. Haaland also estimated the B-N BDE (31.1 kcal mol -1 ) by making comparisons with related methylated aminoboranes 3.…”

Quantum study of the ground state of a series of unsaturated boron–nitrogen compounds by the MNDO method. I. Geometries and stabilities

“…The microwave spectrum of this molecule confirms that it exists in monomeric form, in gas phase, at room temperature, although it presents a strong tendency to polymerize. Table I compares MNDO and ab initio MO results for fully optimization structure [15,16] along with the experimental data. All these results are in good agreement.…”

Effects of Orbital Vacancies in Boron Compounds

“…A previous result, at the 6-3lG level, also favors D2d over D2h, by 11 kcal/mole (14). Extension of the basis sets to the 4-31G level and addition of configuration interaction to the minimum S later set widens the energy gap between the 1103 (staggered) and the less stable 1103 (eclipsed) conformers of B3H7.…”