Ab Initio Investigation of Vibrational Effects on Magnetic Hyperfine Coupling Constants in the X³Σ_g^- State of B₂H₂

Abstract

The influence of the vibrational motion on the magnetic hyperfine coupling constants (hfcc's) in the X 3 Σ gof B 2 H 2 is investigated by means of ab initio methods. The present study clearly shows that for the isotropic hfcc's of the boron centers the incorporation of the vibrational effects is essential already for the vibrational ground state. For the boron center the isotropic hfcc's calculated for the lowest vibrational state (A iso ( 11 B) ) -5.2 MHz) differ considerably from the value obtained at the equilibrium geometry (A iso ( 11 B) ) -14.0 MHz) but agrees excellently with the experimental value (A iso ( 11 B) ) -5.2 MHz) obtained previously. For the hydrogen centers and for all anisotropic hfcc's the effects are found to be much smaller. In the present study the averaged values for various vibrational states and isotopic effects are given.Binary boron species receive more and more attention 1 because the process of their thermal decomposition is potentially important in the creation of thin metallic films involved in the manufacture of semiconductor devices. The HBBH molecule, which represents one of the simplest members of the family of binary boron species, was studied by various theoretical approaches. 2 Very recently, in an excellent study Knight and co-workers 3 described an experimental characterization of HBBH using electron spin resonance (ESR) in neon and argon matrices at 4 K. With the exception of A iso ( 11 B) the measured magnetic hyperfine coupling constants (hfcc's) showed close agreement with ab initio theoretical calculations conducted as a part of their study. For A iso ( 11 B), however, a large discrepancy between the experimental (-5.2(4) MHz) and the theoretical value (-19.1 MHz) was found.In a previous ab initio investigation of the X 2 Π u state of B 2 H 2 + , 4,5 we found that the isotropic hfcc's of the boron center are largely affected by vibronic effects arising from the cis and the trans bending motion. Because the neutral and the ionic systems are very similar, analogous results are also expected for the B 2 H 2 molecule.For the description of the vibrational effects we employ the potential energy surface obtained in a current study 6 that uses the same method as employed in our B 2 H 2 + study. To obtain the electronic hfcc's as a function of the nuclear geometry we employed the AO basis set given by Chipman. 7 The electronic hfcc's are calculated using the MRD CI/B K method. 8 In all calculations of the electronic hfcc's the bond distances are kept fixed at the optimized values of the equilibrium geometry (B-H ) 2.231 bohr; B-B ) 2.869 bohr).At the equilibrium geometry (linear nuclear arrangement) we obtained A iso ( 11 B) ) -14.0 MHz and A iso (H) ) -41.4 MHz. Both values are somewhat different from the theoretical values obtained by Knight and co-workers (A iso ( 11 B) ) -19.

Rydberg and Valence States in the Tetra-atomic Molecules B2H2, C2H2 and C2H2+

“…Although the boron hydrides, particularly the binary boron species, have been the subject of numerous experimental and theoretical studies (see, for example, the reviews by Greenwood [42] and Downs and Pulham [43]), there is very little experimental information about the smallest member of the diborane hydrides family, Only recently Knight et al [44] reported the first definitive experimental characterization of this species by electron spin resonance in neon and argon matrices at 4 K. Controversial results of this study were explained by means of subsequent ab initio calculations [45]. Earlier experimental studies have achieved only fragmentary information concerning and its positive ion.…”

Diborene: Generation and Photoelectron Spectroscopy of an Inorganic Biradical

“…The molecule has only been observed in rare gas matrices, where EPR (electron paramagnetic resonance) spectra indicated a triplet ground state 15 and an IR-spectrum revealed the ν 3 antisymmetric stretch at 2679.9 cm -1 . 16 [19][20][21][22] also focusing on the EPR properties 23 and most recently on the competition between multicenter and classical bonding. 24 However, no computations of the Renner-Tellersplit cation have yet been reported.…”

“…Diborene 1 is a linear molecule of D ∞h symmetry with a 3 Σ g − ground state. 23 The highest singly occupied molecular orbitals (SOMO) are the [1π u (p x )] 1 [1π u p y ] 1 binding orbitals, so the formal bond order is 2. However, because the unpaired electrons are situated in orthogonal MOs, the double bond used in a Lewis-description is questionable in MO theory and therefore omitted from Scheme 1.…”

“…The experiments are complemented by high-level ab initio computations. A large number of theoretical studies are available for the neutral ground state of 1 , − also focusing on the EPR properties and most recently on the competition between multicenter and classical bonding . However, no computations of the Renner–Teller-split cation have yet been reported.…”

Ab Initio Study of the Electronic Spectrum of B2H2

“…study on B 2 H / 2 , including the correlation effects beyond The B 2 H 2 molecule is of great chemical interest because fourth-order perturbation theory. of the X 2 P u state of B 2 H / 2 (13,14) and to compute the with Those of Molecules of Lower Symmetry hyperfine coupling constants in the X 3 S 0 g state of the neutral (15). The electron-deficient nature stable structure had acetylene-like geometry.…”