Comparison of calculated structural parameters and infrared spectra with experimental data for dimeric dibenzyl carbamic acid

doi:10.1016/S0166-1280(02)00465-7

Journal of Molecular Structure: THEOCHEM

Volume 618, Issues 1–2, 15 November 2002, Pages 101-108

https://doi.org/10.1016/S0166-1280(02)00465-7 Get rights and content

Abstract

DFT (B3PW91/6-31G** and B3PW91/6-311G**) calculations were performed for dibenzylcarbamic acid, either monomer or dimer (DBCAD). Six possible structures of the dimer were considered. Calculated geometrical parameters of the most stable dimer were compared with those published for the dibenzylcarbamic acid dimer crystalline solid. The calculated IR spectrum was used for the assignment of the frequencies observed in the experimental IR spectrum of dimer dibenzylcarbamic acid.

Introduction

Recently, the synthesis and X-ray structure of dimer carbamic acid has been reported, that represents the first example of isolated carbamic acid, DBCAD [1]. This prompted us to complete our theoretical studies on the interaction of amines with carbon dioxide. In the recent years, in fact, we have simulated the reaction of formation of dimethylcarbamic acid [(CH₃)₂NCOOH, DMCA] and its reaction product with dimethylamine [2], [(CH₃)₂NCOO^–H₂N(CH₃)₂]. We have also calculated the IR spectra of such species [3], and completed the study with calculations for the reaction of ammonia with carbon dioxide, that leads to the simplest carbamic acid [4], H₂NCOOH. The results of our studies did show that, although carbamic acid hydrogen bond complexes with amines have a smaller stabilisation energy (ca. 60 kJ/mol) than the dimers of carbamic acids (ca. 90 kJ/mol), the isolated products of the reaction of amines with carbon dioxide usually are the carbamic acid ammonium salts. We also showed that the latter are formed from the hydrogen bond complex of carbamic acid with amines [2], [3], [4]. More recently, we have calculated (using the B3PW91/6-31G** and B3PW91/6-311G** methods) the stabilisation energy and IR spectra for a series of carbamic acid dimers and carbamic acid salts with the parent amines [5]. We have shown that the stability of the carbamic acid dimers does not depend on nature of the substituents. Nevertheless, DBCAD is the only carbamic acid dimer isolated so far, despite many attempts have been made with similar systems [1]. This may be due either to a solubility matter or to the fact that the large benzyl substituents in the DBCAD may protect the carbamic acid moiety from further attack by dibenzylamine present also in traces in the reaction medium, or to both. As a matter of fact, the separation of solid DBCAD from the solution may reduce its reactivity.

In this paper we report the results of our studies on six possible conformations of the dimer-dibenzylcarbamic acid by using the density functional theory (DFT) (B3PW91/6-31G** and B3PW91/6-311G**) methods. We also compare the geometrical parameters of the most stable out of the calculated dimers with those published for the dibenzylcarbamic acid dimer crystal [1]. Finally, we interpret the experimental IR spectrum of dibenzylcarbamic acid dimer on the basis of the calculated IR spectrum.

Section snippets

Calculations

The DFT B3PW91 [6], [7] calculations, using both the 6-31G** and 6-311G** basis sets, were performed on a Silicon Graphics Power Indigo 2 (Industrial Chemistry Research Institute (ICRI), Warsaw) and CRAY SV1-1A (Interdisciplinary Mathematical and Computer Modelling (ICM), Warsaw University) computers using the gaussian94 program [8]. The stabilisation energies were corrected for basis set superposition error using two methods: the Boys–Bernardi counterpoise correction [9] and 7-points

Results and discussion

Six structures of dibenzylcarbamic acid dimer are presented in Fig. 1. We shall refer to these structures as trans–trans (TT) (Fig. 1a), inverse trans–inverse trans (ITIT) (Fig. 1b), trans–inverse trans (TIT) (Fig. 1c), cis–cis (CC) (Fig. 1d), inverse cis–inverse cis (ICIC) (Fig. 1e), and cis–inverse cis (CIC) (Fig. 1f). The abbreviations trans, inverse trans, cis, and inverse cis denote the direction of the two benzyl substituents in a single monomer unit of dibenzylcarbamic acid molcule,

Conclusions

In this paper we discuss the results of DFT calculations of six different structures of dibenzylcarbamic acid. We show that the geometry of the most stable structure is in very good agreement with the geometry of the X-ray structure recently published [1]. We have determined that for dibenzylcarbamic acid the dimer stabilisation energy is equal to ca. −86.4 kJ/mol or ca. −73.0 kJ/mol (B3PW91/6-311G**) when corrected for basis set superposition error. The experimental IR spectrum was interpreted

Acknowledgements

This work was supported by the State Committee of Scientific Research in Poland (KBN): Grant No. 3 T09B 050 17 for international co-operation in the COST D9/012/98 project. Results were in part obtained thanks to the Computational Grant No. G19-4 from the Interdisciplinary Centre of Mathematical and Computer Modelling (ICM), Warsaw University, Warsaw. Authors acknowledge M. Aresta for helpful discussion, and L. Labella and D. Belli Dell'Amico for infrared experimental data.

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Comparison of calculated structural parameters and infrared spectra with experimental data for dimeric dibenzyl carbamic acid

Abstract

Introduction

Section snippets

Calculations

Results and discussion

Conclusions

Acknowledgements

J. Mol. Struct.

J. Mol. Struct.

Vib. Spectros.

Vib. Spectros.

Chem. Commun.

J. Chem. Phys.

Phys. Rev.