Structure of the Benzene···ICl Complex: A UVPES and ab Initio Molecular Orbital Study†

A comprehensive statistical approach to identify correct types of cross-peaks based on their symmetric feature

Cross-peak signature patterns in two-dimensional (2D) asynchronous spectra are a useful tool for revealing the deeper physicochemical characteristics of intermolecular interactions. Various interferences such as noise can cause problems in recognizing the correct type of cross-peaks. The symmetrical characteristics of the cross-peaks may offer an intrinsic criterion for distinguishing between different types of cross-peaks. When intermolecular interaction only brings about a shift in the characteristic peak of a solute, we found that the resultant cross-peaks possess a mirror symmetry. However, such a symmetry may not be directly observable in the 2D asynchronous spectrum covered by severe noise. To solve this problem, a comprehensive approach to identify the symmetry is developed. Firstly, a method to locate the mirror is proposed. Then, the Kolmogorov-Smirnov test and Bayesian analysis are used to obtain the probability of the existence of mirror symmetry in the cross-peak group. The approach is showcased in three model systems and a real-world example (the benzene/I2 system), which demonstrates that the approach is helpful in assigning the correct type of cross-peak under difficult situations.

Novel Method for Extracting the Spectrum of a Supramolecular Complex via a Comprehensive Approach Involving Two-Dimensional Correlation Spectroscopy, Genetic Algorithm, and Grid Searching

A supramolecular complex may be formed by two solutes via a weak intermolecular interaction in a solution. The spectrum of the complex is often inundated by the spectra of the solutes that are not involved in the intermolecular interaction. Herein, a novel spectral analysis approach is proposed to retrieve the spectrum of the supramolecular complex. First, a two-dimensional (2D) asynchronous spectrum is constructed. Then, a genetic algorithm is used to obtain a heuristic spectrum of the supramolecular complex. The heuristic spectrum is a linear combination of the spectrum of the complex and the spectrum of a solute. The coefficients of the linear combination are then obtained, according to which the equilibrium constants are invariant among the sample solutions used to construct the 2D asynchronous spectrum. We have applied the approach to a supramolecular system formed by benzene and I₂. In the analysis, several binding models are evaluated, and a benzene molecule interacting with two iodine molecules via halogen bonding turns out to be the only possible model. Hence, the characteristic band of the benzene/I₂ supramolecular complex around 1819 cm^-1 in the Fourier transform infrared (FTIR) spectrum and the corresponding equilibrium constant are obtained. The above results indicate that the novel approach provides a chance to get new insight into various intermolecular interactions studied by spectroscopy.

Ultrasonic, spectrophotometric and theoretical studies of sigma and PI interactions of iodine with substituted benzene

The acoustical method shows the ability to study the structure, energetics and spectroscopic aspects of charge transfer complexes of three different benzenoid compounds with iodine and is supported by the UV-Visible and DFT methods.

MODELING THE GROUND STATE GEOMETRY AND ESTIMATING THE CHARGE TRANSFER TRANSITION ENERGY OF THE TOLUENE–ICl MOLECULAR COMPLEX BY AB INITIO AND DFT METHODS

Out of several plausible isomeric structures of the toluene–ICl charge transfer (CT) complex, the most feasible one was determined by a detailed ab initio and DFT study at the HF, B3LYP, and mPW1PW91 levels using 6-31++G(d, p) basis set. Potential energy surface scans were performed with six possible structures ( I and Cl facing the o-, m-, and p-carbon atoms of toluene separately); the structures at the local minima of the surfaces were subjected to frequency calculation and the ones having no negative frequency were accepted as the real structure in the ground state. These structures were then subjected to full optimization. It was observed that the I – Cl bond, with its I atom oriented toward the aromatic ring, stands vertically above a C -atom at the ortho or para positions, being inclined at about 9° to the line perpendicular to the aromatic ring. Complexation increases the I – Cl bond length. After correction for basis set superposition error through a counterpoise calculation, we conclude from the binding energy that the preferred structure is the one with ICl above the ortho C atom. The calculated binding energy closely matches the experimental free energy of complexation. The electronic CT transition energy (hν CT ) with this structure in the ground state was calculated in vacuo by the restricted configuration interaction singlets method and in carbontetrachloride medium by the time dependent density functional theory method under the polarizable continuum model. The value of hν CT obtained from the ground-to-excited state transition electric dipole moments of the complex, is close to (somewhat underestimated) the reported experimental value.

Resonance Raman Investigation of the Short-Time Photodissociation Dynamics of the Charge-Transfer Absorption of the I2−Benzene Complex in Benzene Solution

Resonance Raman spectra were obtained for the I2-benzene complex in benzene solvent with excitation wavelengths in resonance with the CT-band absorption. These spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion mainly along the nominal I-I stretch mode nu(18), the nominal symmetric benzene ring stretch mode nu5, and the nominal symmetric CCH bending nu7. There is also a small contribution from the nominal out-of-plane CH oop wag nu15. A preliminary resonance Raman intensity analysis was done, and the results for the I2-benzene complex were compared to results previously reported for the 1-hexene-I2 complex. We briefly discuss the differences and similarities in the CT-band absorption excitation of an I2-benzene complex relative to those of an I2-alkene complex.

Molecular structures of cation...pi(arene) interactions for alkali metals with pi- and sigma-modalities

The monovalent cations of Na(+), K(+), Rb(+), and Cs(+) derived from the highly electropositive alkali metals represent prototypical charged spheres that are mainly subject to relatively simple electrostatic and solvation (hydration) forces. We now find that the largest of these Rb(+) and Cs(+) are involved in rather strong cation...pi(arene) interactions when they are suitably disposed with the ambifunctional hexasubstituted benzene C(6)E(6). The ether tentacles (E = methoxymethyl) allow these cations to effect eta(1)-bonding to the benzene center in a manner strongly reminiscent of the classical sigma-arene complexes with positively charged electrophiles where Z(+) = CH(3)(+), Br(+), Cl(+), Et(3)Si(+), etc. The somewhat smaller potassium cation is involved in a similar M(+)...pi(arene) interaction that leads to eta(2)-bonding with the aromatic center in the pi-mode previously defined in the well-known series of silver(I)/arene complexes. We can find no evidence for significant Na(+)... pi(arene) interaction under essentially the same conditions. As such, the sigma-structure of the Rb(+) and Cs(+) complexes and pi-structure of the K(+) complex are completely integrated into the continuum of sigma-pi bondings of various types of electrophilic (cationic) acceptors with arene donors that were initially identified by Mulliken as charge-transfer.