High-Level ab Initio Investigations on Structures and Energetics of N₂O Clusters

Abstract

Both experimental and theoretical investigations on weakly bonded small N2O clusters have been a subject of interest for the past decade. The current article presents high-level ab initio calculations for (N2O)n clusters for n = 4-6 employing second-order Møller-Plesset (MP2) theory and coupled cluster singles and doubles with perturbative triple (CCSD(T)) theory using Dunning's correlation-consistent basis sets. The electrostatics-guided cluster building code developed in our laboratory is applied for the generation of initial cluster geometries, followed by geometry optimization at MP2/aug-cc-pVTZ level of theory. Calculations of single point energy at CCSD(T)/aug-cc-pVTZ and vibrational frequency at the MP2/aug-cc-pVTZ level of theory are facilitated by the fragment-based molecular tailoring approach (MTA). A comparison of the results is done with those obtained by employing dispersion-corrected density functional B2PLYPD with aug-cc-pVTZ basis set. The geometrical parameters and vibrational spectra obtained from these ab initio methods are found to be in good agreement with those derived from recent experimental findings of Oliaee et al. [J. Chem. Phys. 2011, 134, 074310] and Rezaei et al. [J. Chem. Phys. 2012, 136, 224308].

Accurate vibrational spectra via molecular tailoring approach: A case study of water clusters at MP2 level

“…Since, the dipole derivatives in normal modes are the only molecular properties entering the formula, the intensity, thus, derived is termed as the absolute IR intensity in vibrational spectra analyses. 57,58 The above mentioned algorithm including the grafting procedure for the Hessian matrix and dipole derivatives along with the calculation of vibrational spectra through normal modes has been applied to three isomers of (H 2 O) 16 12 are fully optimized (feasible without fragmentation using off-theshelf hardware) employing MP2/aDZ and MP2/6-311+G * * levels of theory, respectively, whereas the cationic clusters are optimized at MP2/aDZ level of theory using MTA. All the test systems are displayed 59,60 in Figure 1, and the corresponding Cartesian coordinates are given in the supplementary material (Table S1-S9).…”

“…49,50,56 These grafted values of the Hessian and dipole derivatives are further utilized for the estimation of accurate vibrational spectra of molecular systems via the normal mode analyses. This method for estimating the vibrational spectra has been tested out on both neutral [(H 2 O) 16 12 ] employing either MP2/aug-cc-pVDZ or MP2/6-311+G * * level of theory. The calculated results are benchmarked against their fully calculated counterparts for neutral water clusters and compared with the available experimental results for charged clusters, bringing out excellent agreement for all the cases.…”

“…3 Several studies, reporting the theoretically calculated vibrational spectra of a variety of molecular clusters, are available in the literature. [4][5][6][7][8][9][10][11][12][13] Among these, most of the investigations are reported on water clusters. Xantheas and coworkers [4][5][6][7] have explored the energetics and vibrational spectra of water clusters of various sizes, during the last decade.…”

“…35 On the other hand, generalized energy-based fragmentation (GEBF) approach 21 has been widely used for the geometry optimization and vibrational spectra of a variety of large molecular systems including peptides, zwitterions, water clusters. 36 Both Raman and infrared (IR) spectra for (H 2 O) 28 and (Glycine) 12 , gelatin A and gelatin B were reported at HF/6-31G* level of theory. 36 However, for a better understanding of the molecular features as well as for accurate energetics, highlevel calculations such as MP2 with relatively large basis sets are necessary and desirable.…”

“…An automated FORTRAN code has been developed for the accurate estimation of vibrational frequencies, intensities, and the assignment of the former based on normal mode analysis. In the present work, the proposed algorithm is tested out on three isomers of (H 2 O) 16 12 cluster at MP2/6-311+G * * level of theory are compared to the corresponding experimental findings and/or theoretically calculated spectra. [51][52][53][54][55] …”