8648 J. Phys. Chem. 1990, 94, 8648-8655 ARTICLES Experimental Orbital Momentum Distributions and Bonding Interactions. 1. Electron Donor-Acceptor Complex (CH,),N-BF, Kathleen McMillan,t Michael A. Coplan,*st John H. Moore,$ and John A. Tossellt itutitutr ,lor Physicol Science and Twhnologj, and Dtpurtment of Chemistrj, and Biochemistry. University of z44urj~lund. CollPge Park, Maryland 20742 {Receired: December 14. 1989: In Final Form: April 27. 1990) A comparison of the experimental momentum distribution of electrons in the highest occupied molecular orbital of N(CH,), with the electron momentum distribution of the boron-nitrogen bonding orbital of (CH,),N-BF, shows that the orbital of the complex has a larger relative density of high momentum electrons. This increase in high momentum components is associated nith the incrcasc in nodal surfaces in the orbital of the complex in comparison with the orbital of the amine. Qualitative agreement is seen in a comparison of the experimental momentum distributions with distributions calculated from small basis set ab initio position space orbital wave functions. The experimental results are also discussed in terms of the position apace autocorrelation function diffcrcncc M(r). which is demonstrated to provide additional information concerning the orbital interactions. 1. introduction Thc bcn\iti\,it! and uccuracq with which (e,2e) spectroscopy can bc applied to the measurement of valence electron momentum distributions in gas-phase molecules by the present generation of high momentum resolution spectrometers has been well established during recent years.'-3 The basis of the spectroscopic technique is the observation of the high energy electron knock-out. or (e,2e), reaction. When the binding energy of electrons in a particular niolecular orbital differs sufficiently from the binding energy of othcr electrons in the molecule, it is possible to observe (e&) reactions involving essentially only the electrons of that orbital. The experimentally obtained momentum distribution p(p) is then the <phcrical average of the square modulus of the momentum space orbital wave function $ (p). In the work reported here, we have initiated an (e,2e) spectroscopic study of bonding interactions between molecules. Theoretical treatments of chemical reactivity and bonding have demonstrated that molecular interactions are often dominated by the frontier orbitals of the reactant molecule^.^ (c.7~) rpectroscopy. becnusc it can measure orbital specific mo- mentum distributions. can provide information on such phenomena 215 clcctroii rcdihtribution and rchybridization associated with intcractiona of l'ronticr orbitals. The subject of this invcstigation is the reaction of boron tri- fluoride (BF,) and trimethylamine (N(CH,),) to form the electron donor-acccptor complex (CH,),N-BF,. Formation of the in- termolecular bond involves the donation of charge density from the iionbondiiig highest occupied orbital of thc donor nioleculc Y(Cl13)3 to the antibonding T* lowest unoccupied orbital of the Lcni\ acid RF,. The transfer of electron density from the donor lea& to ;I nenkening of the bonds within the BF, moiety. and the complcu (CH1)?N-BF3 is therefore the product of what has been tcrmcd .in "incipient displacement r e a c t i ~ n " . ~ , ~ Our approach to tlic btud) of the bonding interaction is to mcasurc thc nio- mciituiii di\tributions of two molecular orbitals: (i) the nonbonding hiyhcbt occupicd molecular orbital (HOMO) of Y(CH,), and (ii) thc boron-nitrogen bonding orbital of the complex complex (CH,),N-BF,. The orbital of ii is the HOMO of the complex and correlates with the HOMO of N(CH,), and the lowest unoccupied molecular orbital (LUMO) of BF,. By comparing the t\ro momcntum diytributions. the rearrangement in momentum _- ' Inslitutu for Ph!\ic,il Sciencc and Tcchnologq : Dcpnritiicnl of Chctnisirl Jnd Biochemisirk. space of a frontier orbital of a reactant molecule can be directly observed. A review of the theoretical foundation for the application of (de) spectroscopy to the determination of orbital momentum distributions is provided in the next section. Section Ill describes the experimental method. A calculation of the binding energies of (CH3),N-BF3 and the correlation of the molecular orbitals of the complex with those of N(CH3), and BF, are discussed in section 1V.A. The experimental momentum distributions for the HOMO's of IV(CH,), and (CH,),N-BF, are presented and compared in section 1V.B. The experimental distributions are also compared with theoretical distributions calculated from small basis set SCF-MO position space wave functions for the reactant amine and the complex orbitals. In section 1V.C the spherically averaged position space autocorrelation function B(r) is calculated from the experimental momentum distributions for the Y(CH,), and (CH3),N-BF, HOMO's and is used to quantify differences in position spacc between thc experimental electron distributions of the two orbitals. Some conclusions regarding intermolecular bonding in (CH3),N-BF3 arc presentcd in section V. 11. Theoretical Background In an (e,2e) reaction, a target atom or molecule M is struck b) an electron c, with sufficient kinetic energy E, to knock out an electron from the target: (1) The kincniatic parameters defining the reaction for a target at rest are the momenta k,, k,,,. and k, of the incident and outgoing electrons. and the recoil momentum q of the residual ion. Al- though the outgoing electrons eAand eBmay leave the scattering center with any energies and momenta that satisfy the conservation hws. constraints are imposed experimentally on those electrons that can be detected and observed. In the noncoplanar symmetric gcometr) of our experiment (Figure I), only those electrons eA e, + M - M+ + cA + eB (I) Bawagan, A. 0.; Muller-Fiedler. R.; Brion, C. E.; Davidson, E. R.: (2) Goruganthu, R. R.: Coplan, M. A.: Leung. K. T.: Tossell. J. A,; Moore, (3) Grisogono, A. M.; Pascual. R.: Von Niessen, W.; Weigold. E. Chem. (4) Fukui. K.; Koga, N.: Fujimoto. H. J. Ant. Chem. Soc. 1981. 103, 196. (5) Jenacn, W. B. The Lewis ilcid-Base Concepts: Wile~-lnterscience: (6) Bcnl. H. A. C'henl Rev. 1968. 68, 587. Boyle, C . Chem. Phys. 1988, 120, 335. J. H. J. Chem. Phys. 1989, 91, 1994. Phj,.c.. 1989. 135. 317. VCN York. 1980 0022-3654/90/2094-8648$02.50/0 0 1990 American Chemical Society