Towards a Spectroscopic and Theoretical Identification of the Isolated Building Blocks of the Benzene–Acetylene Cocrystal

Abstract

Isomer- and mass-selective UV and IR-UV double resonance spectra of the BA3, B2A, and B2A2 clusters of benzene (B) and acetylene (A) are presented. Cluster structures are assigned by comparison with the UV and IR spectra of benzene, the benzene dimer, as well as the BA, BA2, and B2A clusters. The intermolecular vibrations of BA are identified by dispersed fluorescence spectroscopy. Assignment of the cluster structures is supported by quantum chemical calculations of IR spectra with spin-component scaled second-order Møller-Plesset (SCS-MP2) theory. Initial propositions for various structures of the BA3 and B2A2 aggregates are generated with model potentials based on density functional theory combined with the symmetry-adapted perturbation theory (DFT-SAPT) approach. Shape and relative cluster stabilities are then confirmed with SCS-MP2. T-shaped geometries are the dominant structural motifs. Higher-energy isomers are also observed. The detected cluster structures are correlated with possible cluster formation pathways and their role as crystallization seeds is discussed.

Benzene⋯acetylene: a structural investigation of the prototypical CH⋯π interaction

“…These would originate from two doubly degenerate ''bending'' modes and a singly degenerate stretching mode, corresponding to the two rotational and three translational degrees of freedom of free HCCH. Bo ¨ning et al 15 used dispersed fluorescence to determine intermolecular vibrational frequencies for BZÁ Á ÁHCCH of 62.4 cm À1 , 94 cm À1 and 131.7 cm À1 , for degenerate hindered rotation of HCCH relative to BZ, degenerate translation of HCCH relative to BZ, and non-degenerate translation of HCCH perpendicular to BZ, a For the parent species. b 13 C closest to the benzene ring.…”

“…These interactions have been known for many years to be important in a wide range of chemical circumstances, and they were first noted as long ago as 1952, with Tamres's calorimetric measurements in solutions of chloroform with aromatic compounds. 1 Experimentally, most examples of aromatic CHÁ Á Áp interactions have been identified in crystal structures or by IR or NMR spectroscopy [2][3][4] (although some gas phase studies are also available 2,3,[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] ) with the most often studied interactions involving an alkyl hydrogen atom oriented towards the center of an aromatic ring. 20 Most spectroscopic investigations have not attained sufficient resolution to obtain detailed structural information about the clusters, [4][5][6][7][8][9]12,15,19 although high resolution (rotational spectroscopy) studies of fluorobenzeneÁ Á ÁHCCH, 18 BZÁ Á ÁHCF 3 , 13,14 and benzene dimer 16,17 are available.…”

“…1 Experimentally, most examples of aromatic CHÁ Á Áp interactions have been identified in crystal structures or by IR or NMR spectroscopy [2][3][4] (although some gas phase studies are also available 2,3,[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] ) with the most often studied interactions involving an alkyl hydrogen atom oriented towards the center of an aromatic ring. 20 Most spectroscopic investigations have not attained sufficient resolution to obtain detailed structural information about the clusters, [4][5][6][7][8][9]12,15,19 although high resolution (rotational spectroscopy) studies of fluorobenzeneÁ Á ÁHCCH, 18 BZÁ Á ÁHCF 3 , 13,14 and benzene dimer 16,17 are available. These provide structural information on the isolated weak CHÁ Á Áp contact without the possible influence and constraints of crystal packing.…”

FFCASP: A Massively Parallel Crystal Structure Prediction Algorithm

“…In this regard, Symmetry-Adapted Perturbation Theory (SAPT) combined with DFT (SAPT­(DFT) or DFT-SAPT − ), as accurate as single and double excitation coupled cluster theory including noniterative triple excitations (CCSD­(T)), is an approach used to create accurate intermolecular force fields. In particular, both SAPT­(DFT) and DFT-SAPT have been successfully used to develop tailor-made energy functions for benzene, , acetylene, acetylene–benzene, and deoxyribonucleic acid (DNA) bases. − More importantly, Podeszwa et al used SAPT­(DFT) dimer force fields to predict the crystal structures of cyclotrimethylene trinitramine (RDX) and benzene with the help of a molecular packing and lattice energy minimization method, MOLPAK/WMIN code, and showed that SAPT­(DFT)-based force fields lead to reliable generation of crystal structures of these organic molecules.…”

Effect of aromatic ring fluorination on CH⋯π interactions: rotational spectrum and structure of the fluorobenzene⋯acetylene weakly bound dimer

“…12 The relatively acidic proton in acetylene makes this molecule a good probe for investigating CHÁ Á Áp interactions, and the prototype system, benzene(BZ)Á Á ÁHCCH has been studied extensively by theoretical and experimental techniques. [13][14][15] X-ray studies of the 1 : 1 cocrystal of benzene with acetylene 16 revealed a timeaveraged position of each HCCH molecule between parallel benzene rings with its axis perpendicular to the aromatic planes. These HCCHÁ Á Áaromatic interactions should be distinguished from the interactions between aromatic p systems and alkyl hydrogen atoms, since acetylene is often described as containing an ''activated'' CH bond, which is more acidic than those of, for instance, ethylene or methane (pK a values: 17 25 (acetylene), 45 (ethylene), 59 (methane)).…”