Structural and Energetic Properties of Haloacetonitrile–BCl₃ Complexes: Computations and Matrix-IR Spectroscopy

Abstract

The FCHCN-BCl and ClCHCN-BCl complexes were investigated by quantum-chemical computations and low-temperature, matrix-isolation-IR spectroscopy. Theory predicts two stable equilibrium structures, with distinctly different B-N distances, for both complexes. One set of structures, which correspond to the global energy minima, exhibit B-N distances of 1.610 and 1.604 Å for FCHCN-BCl and ClCHCN-BCl, respectively (via M06-2X/aug-cc-pVTZ). The corresponding binding energies are 5.3 and 6.3 kcal/mol. For the metastable structures, the B-N distances are 2.870 and 2.865 Å for FCHCN-BCl and ClCHCN-BCl, respectively, and the corresponding binding energies are 3.2 and 3.3 kcal/mol. Also, the barriers between these structures on the B-N distance potentials are 2.5 and 2.8 kcal/mol, respectively, relative to the secondary, long-bond minima. In addition, several IR bands of both FCHCN-BCl and ClCHCN-BCl were observed in nitrogen matrices, but the assigned bands are consistent with M06-2X predictions for the short-bond, minimum-energy structures. None of the observed IR bands could be assigned to the metastable, long-bond structures.

Structural and energetic properties of RMX3‐NH3 complexes

“…In fact, the CH 3 SiCl 3 –NH 3 and CH 3 GeCl 3 –NH 3 systems exhibit two distinct R‐axial structures with distinctly different M‐N bond distances, a phenomenon we have encountered before in our studies of nitrile‐BCl 3 complexes. [ 39,40 ] For the most part, these structures lie 2 to 3 kcal/mol above the respective R‐equatorial structures. The exceptions are the short‐bond forms of the RMCl 3 –NH 3 complexes, which lie about 4 to 6 kcal/mol above their R‐equatorial counterparts.…”

Boron Tunneling in the “Weak” Bond‐Stretch Isomerization of N−B Lewis Adducts

“…We found that M06-2X/6-311 + G(d) meets the requirements, providing a close agreement with structural parameters; the diffuse functions were included to improve the accuracy of charge transfer interactions. [27,29,30,38] Classical (i. e. non-QMT) reaction rates were computed using canonical variational transition state theory (CVT), [39] with QMT contributions added with the small curvature tunneling (SCT) [40,41] method; a step size of 0.001 Bohr and quantized reactant state tunneling (QRST) for the reaction coordinate mode were used. [42] The M06-2X/6-311 + G(d) electronic energies were corrected with a double layer method (the interpolated single-point energy -ISPE- [43] ), where the energies of reactants, products, and transition states were recomputed at the CCSD(T)/CBS level (obtained from aug-cc-pVTZ and aug-cc-pVQZ extrapolation with β coefficients of 5 and 3 for the HF and post-HF correlation, except for Br where an aug-cc-pvTZ-PP and aug-cc-pvQZ-PP basis set was employed).…”

“…The barriers between these two minima lie between 4 to 11 kJ mol −1 with respect to the meta‐stable LB, and their corresponding reaction energies are in the range of 5–20 kJ mol −1 [29,30] . This would, in principle, make it possible to observe both geometries if a high‐temperature mixture of the two configurations is fast‐cooled to cryogenic conditions, keeping the system in the out‐of‐equilibrium original state by its kinetic barrier.…”

“…However, in stark contrast with this assumption, matrix-isolation IR spectra revealed only the presence of the short-bond structure with the authors speculating that the elusive long-bond form might be masked by HCl impurities forming H-bonded adducts. [29,30] In this work, we propose that the reason for the unsuccessful attempts of capturing and detecting the LB structure even close to the absolute zero can be attributed to heavy-atom quantum mechanical tunneling (QMT). QMT is a well-known effect for hydrogen-based reactions, but only recently it was established that "heavy" atom tunneling (i. e. second-row atoms) can also play an important role for reactions with low and, most importantly, narrow barriers.…”

Twenty years of exceptional success: The molecular education and research consortium in undergraduate computational chemistry (MERCURY)

“…The faculty published 2 peer‐reviewed books, [ 238,239 ] 9 peer‐reviewed book chapters, [ 240–248 ] and 115 peer‐reviewed research papers. [ 249–363 ] This comes to 1.6 peer‐reviewed products/faculty/year during the 3‐year grant period, which is 3.2 times the rate of publication for natural science faculty at PUIs. [ 46 ]…”