Anharmonic treatment of vibrational resonance polyads—the diborane: a critical case for numerical methods

Abstract

Based on accurate computational results, the IR spectra of diborane B 2 H 6 and its deuterated derivative B 2 D 6 were experimentally revisited to reconsider or complete their band assignments. A pure, variational approach, developed in both mechanical and electrical anharmonicities, was applied to study the diborane molecule for which many uncertainties remain in the spectral IR assignment. This work, together with all the experiments on this system over recent decades, shows the difficulty of interpreting the spectral data, making it a ''benchmark'' ideal for testing the mathematical approaches for the implementation of vibrational codes.

Theoretical Investigation of the Infrared Spectrum of 5‐Bromo‐2,4‐pentadiynenitrile from a CCSD(T)/B3LYP Anharmonic Potential

“…It should be noted that the corresponding modes in the dicya-noacetylene [6] are computed in the same spectral region between 2064 and 2237cm À1 .T he experimental spectra show the existence of an additional unassigneda symmetric band between 2209 and 2254 cm À1 centered around 2230-2235 cm À1 .B ased on our computeds pectra this intermediate asymmetricb road and weak absorption can be assigned to the three complexc ombinations n 5 + n 6 + n 8 (2245 cm À1 ; 2.9 km mol À1 ), 2n 1 + n 9 (2247cm À1 ;3 .4 km mol À1 )a nd n 2 + n 4 + n 5 + n 8 (2250 cm À1 ;1km mol À1 ). In the same spectralr egion, our theoretical study predicts two bands with intensities close to 1kmmol À1 calculated at 2361 and 2454 cm À1 and, respectively assigned at the combination 2n 6 + n 8 and at the first overtone of the chain stretching mode 2n 8 .I ts hould be noted that this last overtone does not lead, as in the dicyano/acetylene case, to aF ermi resonance with the CNs tretching mode.…”

“…The accuracy of the methods used, has been enhanced by considering the irreducible representations involved, allowing the diagonalization of larger vibrational spaces. Our variational and variational–perturbational approaches, have already allowed the complete assignment of many observed spectra or, better yet, arisen from new experimental studies by revisiting their IR spectra . Our aim is to provide an accurate roadmap of the computed IR spectrum of BrC 5 N that shall guide spectroscopists to identify and highlight low‐intensity bands in regions that have not been completely studied so far for this molecule.…”

“…The accuracy of the methods used, has been enhanced by considering the irreducible representationsi nvolved, allowing the diagonalization of larger vibrational spaces. Our variational andv ariational-perturbational approaches, have already allowed the complete assignment of many observed spectra [6][7][8][9][10][11][12][13] or,b etter yet, arisen from new experimental studies by revisiting their IR spectra. [14][15][16] Our aim is On the grounds of ah ybrid CCSD(T)/B3LYP/aug-cc-pVTZ anharmonic potential and the use of av ariational and variationalperturbational methods, the IR spectrao f5 -bromo-2,4-pentadiynenitrile (BrC 5 N) is revisited in the mid-infrared region up to 4500 cm À1 .Aposition andi ntensity analysis of our theoretical results allow us to assign the fundamental bands together with their combinationsa nd overtones, in the aforementioned range of frequencies.…”

A theoretical study of the spectroscopic properties of B2H6 and of a series of BHyz−species (x = 1−12, y = 3−14, z = 0−2): From BH3 to B12H

“…It has a ring-type molecular structure and has two-bridging hydrogens [63] (see Figure 1). Begue [64] used high-level calculations to investigate the anharmonic vibrational properties of this compound.…”

Accurate Vibrational Configuration Interaction Calculations on Diborane and Its Isotopologues

“…This has recently prompted other groups to reinvestigate the vibrational spectrum. 21,23 At the same time, ab initio methods were employed to compute the vibrational spectrum, but most calculations were limited to the harmonic approximation, which is of limited help in this challenging and strongly resonating system. 16−20 In 2012, Begue et al 21 used variational and combined variation− perturbational approaches relying on a quartic potential to overcome this limitation.…”