Ab initio studies on hydrogen-bonded clusters. I. Linear and cyclic oligomers of hydrogen cyanide

Large-scale ab initio calculations have been performed on linear and cyclic oligomers of hydrogen cyanide molecules applying basis sets ranging from double-zeta to near-Hartree-Fock quality. Equilibrium geometries of linear (HCN)_n clusters with n = 1 to 5 and of cyclic clusters with n = 3, 4 are reported. For most of the complexes complete vibrational analysis has been carried out. In agreement with recent experimental data the linear HCN trimer was found to be more stable than the cyclic trimer. In case of the tetramer linear and cyclic structures are of comparable stability. The structural changes taking place upon polymerization of linear HCN clusters and the convergence of various stage properties to those of the infinite polymer (HCN)_∞ are discussed in detail. The evolution from vibrational spectra of small oligomers to phonon dispersion curves of the infinite polymer is illustrated too.     

Intermolecular vibrations of the water trimer, a matrix isolation study

Infrared spectra from 25 to 4000 cm(-1) have been recorded of water (H2O, D2O and H218O) matrix isolated in neon, argon, and krypton matrices. Intermolecular absorption bands of different isotopologues of the water trimer and tetramer have been assigned from concentration dependencies and diffusion behavior, using the well-known mid-infrared trimer and tetramer absorption bands as measures of the trimer and tetramer concentrations. The results are compared to ab initio calculations.     

DFT predictions of vibrational spectra of titanium tetramethoxide oligomers and the structure of titanium tetraalkoxides in liquid and solid phases

Plausible structures of the titanium tetramethoxide trimer were optimized at the B3LYP/6-31+G^* level. From the four types of structures of the Ti₃O₁₂ cage found earlier for the Ti(OH)₄ trimer only two isomers were found as energy minima on the Ti₃(OMe)₁₂ potential energy surface. One isomer (I), belonging to the C_i point group, is built from three interconnected titanium oxide tetrahedra with a linear arrangement of titanium atoms. This structure have a titanium oxide skeleton similar to that of Ti₃(OH)₁₂. The other isomer (II), of C₂ symmetry, is built by edge sharing TiO₆ groups. Theoretical IR spectra of these isomers are compared with reported experimental IR spectra of solid titanium tetramethoxide and newly obtained Raman spectra of commercial powders. It was shown that the number and position of observed bands in the CO stretching region of the IR spectra of the so-called modification A of solid titanium tetramethoxide are in a good agreement with the predicted vibrational spectrum of trimer I. The equilibrium structure and IR and Raman spectra were also obtained for the Ti₄(OMe)₁₆ tetramer. The comparison of the predicted vibrational spectrum with the experimental IR spectra of modification B as well as of the Raman spectrum of solid titanium tetramethoxide allows us to confirm the tetrameric structure of this modification and to propose the similar structure for commercial samples.     

Synthesis,Structure, Optical,and Electrochemical Properties of Triple‐ and Quadruple‐Decker Co‐facial Tetrathiafulvalene Arrays

Understanding the details of the electronic structure in face‐to‐face arranged tetrathiafulvalenes (TTFs) is very important for the design of supramolecular functional materials and superior conductive organic materials. This article is a comprehensive study of the interactions among columnar stacked TTFs using trimeric (trimer) and tetrameric (tetramer) TTFs linked by alkylenedithio groups (‐S(CH₂)_nS‐, n=1–4) as models of triple‐ and quadruple‐decker TTF arrays. Single‐crystal X‐ray analyses of neutral trimeric TTFs revealed that the three TTF moieties are oriented in a zigzag arrangement. Cyclic voltammetry measurements (CV) reveal that the trimer and tetramer exhibited diverse reversible redox processes with multi‐electron transfers, depending on the length of the ‐S(CH₂)_nS‐ units and substituents. The electronic spectra of the radical cations, prepared by electrochemical oxidation, showed charge resonance (CR) bands in the NIR/IR region (1630–1850 nm), attributed to a mixed valence (MV) state of the triple‐ and quadruple‐decker TTF arrays. In the trimeric systems, the dicationic state (+2; 0.66 cation per TTF unit) was found to be a stable state, whereas the monocationic state (+1) was not observed in the electronic spectra. In the tetrameric system, substituent‐dependent redox processes were observed. Moreover, π‐trimers and π‐tetramers, which show a significant Davydov blueshift in the spectra, are formed in the tricationic (trimer) and tetracationic (tetramer) state. In addition, these attractive interactions are strongly dependent on the length of the linkage unit.     

Quantum chemical exploration of formaldehyde clusters (H2CO)n (n = 2–4)

Global exploration of equilibrium structures and interconversion pathways on the quantum chemical potential energy surface (PES) is performed for (H₂CO)n (n = 2–4) by using the Scaled Hypersphere Search‐Anharmonic Downward Distortion Following (SHS‐ADDF) method. Density functional theoretical (DFT) calculations with empirical dispersion corrections (D3) yielded comparable results for formaldehyde dimer in comparison with recent detailed studies at CCSD(T) levels. Based on DFT‐D3 calculations, trimer and tetramer structures and their stabilities were studied. For tetramer, a highly symmetrical S₄ structure was found as the most stable form in good accordance with experimentally determined tetramer unit in the formaldehyde crystal. © 2018 Wiley Periodicals, Inc.     

Synthesis and polymerization of coupled compounds of 1,11-dodecadiyne

The dimer, trimer, and tetramer of 1,11-dodecadiyne, HC?C? (CH₂)₈? C?CH, were synthesized. The solid-state polymerization of the dimer was investigated by infrared (IR) spectroscopy. IR bands due to the diacetylene moiety were identified through the comparison of the IR spectra of the dimer, trimer, and tetramer. The dimer was found to have two polymorphs, melt-crystallized and solution-crystallized. Both of the polymorphs undergo solid-state polymerization by exposure to γ-ray or UV irradiation. The former has higher polymerizability for the diacetylene moiety than the latter. The solid-state polymerization of the terminal acetylene group was not observed. It is shown that the previously reported dimer structure in which both the diacetylene and terminal acetylene groups are polymerized to form an inherently electrically conducting polymer is incorrect. © 1995 John Wiley & Sons, Inc.     

Growth pattern and electronic properties of acetonitrile clusters: a density functional study

We report a systematic theoretical study on the growth pattern and electronic properties of acetonitrile clusters [(CH(3)CN)(n) (n = 1, 9, 12)] using density functional approach at the B3LYP6-31++G(d,p) level. Although we have considered a large number of configurations for each cluster, the stability of the lowest energy isomer was verified from the Hessian calculation. It is found that the lowest energy isomer of the dimer adopts an antiparallel configuration. For trimer and tetramer, cyclic ring structures were found to be favored over the dipole stabilized structure. In general, it is found that the intermolecular CH...N interactions play a significant role in the stabilization of the cyclic layered geometry of acetonitrile clusters. A critical comparison between trimer and tetramer clusters suggests that the three member cyclic ring is more stable than four member rings. The growth motif for larger clusters (n = 5-9, 12) follows a layered pattern consisting of three or four membered rings, which, in fact, is used as the building block. Based on the stability analysis, it is found that clusters with an even number of molecular entities are more stable than the odd clusters, except trimer and nonamer. The exceptional stability of these two clusters is attributed to the formation of trimembered cyclic rings, which have been found to form the building blocks for larger clusters.     

The Photoelectron Spectra of 4, 13-Diaza- and 4, 16-Diaza [24] (1,2,4,5)cyclophanes

A tentative assignment is proposed for the He (Ia) photoelectron (PE.) spectra of the title compounds. It is based on the assumption that the first four π-bands occupy the same position in both spectra, as suggested by a qualitative correlation with the PE. spectrum of [2₄](1,2,4,5)cyclophane and by a previously proposed molecular orbital model. If this is accepted, then the bands due to electron ejection from the lone-pair orbitals are split by roughly 0.8 eV in the case of the 4, 13-diaza isomer, whereas no split can be detected in the case of the 4, 16-diaza isomer.     

Studies in hydrogen bonding: Association in methanol using an empirical potential

The empirical potential, EPEN , has been used to establish the structures of isolated hydrogen-bonded clusters in methanol. The most stable configuration of the dimer is found to have a trans near-linear form, whereas the most stable forms of the trimer and tetramer are cyclic. Charge interactions in the tetramer make it the most stable, in terms of energy per hydrogen bond, of these three species. These results are in conformity with various types of experiment. Other species of dimer, trimer, and tetramer, corresponding to local energy minima, have also been identified.     

Vibrational and electronic absorption spectra of 4-chloro 2-methyl, 4-chloro 3-methyl and 6-chloro 3-methyl phenols

The infrared spectra of 4-Cl 2-Me, 4-Cl 3-Me and 6-Cl 3-Me phenols have been recorded. The vibrational spectrum has been analysed assuming that the molecules belong toC _s point group and a tentative assignment of the observed frequencies to various modes of vibration has been proposed. The near ultraviolet absorption spectrum of these compounds has also been recorded. Assuming the transition to be electronically allowed the strongest band on the longer wavelength side has been assigned as the (0, 0) band in each case. The spectrum has been analysed in terms of several excited state frequencies which have been correlated with the ground state frequencies observed in the infrared spectrum.     

Geometries, thermodynamic properties and reactions of methylzinc alkoxide clusters studied by density functional theory calculations

Methylzinc alkoxide complexes are precursors for the preparation of nanosized zinc oxide particles, which in turn are catalysts or reagents in important industrial processes such as methanol synthesis and rubber vulcanization. We report for the first time the structures, energies, atomic charges, dipole moments, and vibrational spectra of more than 20 species of the type [(MeZnOR')n] with R' = H, Me, tBu and n = 1-6, calculated by density functional theory methods, mostly at the B3LYP/6-31+G* level of theory. For R' = Me, the global minimum structure of the tetramer (n = 4) is a highly symmetrical heterocubane but a ladder-type isomer is by only 70.9 kJ mol(-1) less stable. The corresponding trimer is most stable as a rooflike structure; a planar six-membered ring of relative energy 13.5 kJ mol(-1) corresponds to a saddle point connecting two equivalent rooflike trimer structures. All dimers form planar four-membered Zn2O2 rings whereas the monomer has a planar CZnOC backbone. A hexameric drumlike structure represents the global minimum on the potential energy hypersurface of [(MeZnOMe)6]. The enthalpies and Gibbs energies of the related dissociation reactions hexamer --> tetramer --> trimer --> dimer --> monomer as well as of a number of isomerization reactions have been calculated. The complexes [(MeZnOMe)n] (n = 1-3) form adducts with Lewis bases such as tetrahydrofuran (thf) and pyridine (py). The binding energy of py to the zinc atoms is about 65% larger than that of thf but is not large enough to break up the larger clusters. The bimolecular disproportionation of [(MeZnOMe)4] with formation of the dicubane [Zn{(MeZn)3(OMe)4}2] and Me2Zn is less endothermic than any isomerization or dissociation reaction of the heterocubane, but for steric reasons this reaction is not possible if R' = tBu. A novel reaction mechanism for the reported interconversion, disproportionation and ligand exchange reactions of zinc alkoxide complexes is proposed.     

Assignment of Phosphorus NMR Parameters to Absolute Configurations in Chiral Phosphorus Sulfide Cage Compounds,using Ab Initio Methods

The closo cage molecules α‐ and β‐P₄S₃(μ‐NCH(Me)Ph) were modelled at the RHF/3‐21G* and MPW1PW91/DZVP levels. For each, the conformational space corresponding to rotation about the C–N bond was explored, and relative average electronic energies were calculated. The β‐isomer was more stable than the α‐isomer by 10.7 kJ mol^?1, according to the DFT calculations, in contrast to the unsubstituted model compounds α‐ and β‐P₄S₃(μ‐NH), where the α‐isomer was more stable. GIAO calculations of phosphorus isotropic NMR shieldings, in the rotamers, led to relative average chemical shifts in the diastereomers. Comparison with experimental chemical shift differences gave an assignment to absolute configuration for α‐P₄S₃(μ‐NCH(Me)Ph), which agreed with the assignment obtained by comparing calculated relative diastereomer stability with observations. For β‐P₄S₃(μ‐NCH(Me)Ph), the GIAO calculations allowed relative assignment of observed chemical shifts to the nitrogen bridgehead phosphorus atoms.     

Notiz über die Photoelektronen-Spektren des Nortricyclens und des Triasterans

The photocelectron spectra of nortricyclene (=tricyclo[2.2.1.0^2,6]heptane) and of triasterane (=tetracyclo[3.3.1.0^2,8.0^4,6]nonane) have been recorded and a tentative assignment of the bands has been put forward on the basis of MINDO/2 SCF calculations.     

Theoretical investigation of the molecular structure of aluminium triisopropoxide and its complexes in ring-opening polymerization

We present a theoretical study on aluminium triisopropoxide (Al(OⁱPr)₃) using both empirical (Molecular Mechanics, MM, with Dreiding II force field) and quantum-chemical (Austin Model 1, AM1, semiempirical Hartree-Fock) techniques. We determine the most stable geometries for both the tetramer and trimer of aluminium triisopropoxide as well as the thermodynamic characteristics of the equilibrium existing between these two aggregated structures. The theoretical results are compared to experimental data from X-ray diffraction and ²⁷A1 NMR measurements. For the tetramer, it appears that the optimal equilibrium geometries are in good agreement with the experimental X-ray diffraction geometry; another geometry is also obtained with both theoretical approaches, which is slightly less stable but of higher symmetry. On the basis of the most stable configurations for the tetramer and trimer aggregates, the variation of free enthalpy (ΔG) between the two aggregated structures has been estimated. The evolution of the theoretical ΔG values indicates a displacement of equilibrium towards the trimer species with temperature, in good agreement with experimental ¹H and ²⁷A1 NMR data. Moreover, the AM1 heats of formation show a gain of 33.9 kcal/mol due to the aggregation of four A1(OⁱPr)₃ instead of three, and thus a better stability of the tetramer. The molecular geometries being well described by the theoretical methods used in this study, we also present a model for the ring-opening polymerization complexes of ε-caprolactone and lactides.     

Dissociation of tetrameric ions of noncovalent streptavidin complexes formed by electrospray ionization

The noncovalent tetrameric association of the protein streptavidin formed by electrospray ionization (ESI) mass spectrometry has been observed intact and dissociated in the gas phase. An extended mass-to-charge ratio range quadrupole mass spectrometer was employed to examine the effects of harsher conditions in the ESI atmosphere-vacuum interface region on the streptavidin tetramer. Thermally induced dissociation caused the mass spectra to exhibit a series of complementary monomer and trimer ions that correspond to decomposition of the tetrameric species. Similar results were obtained with tandem mass spectrometric experiments on a Fourier transform ion cyclotron resonance mass spectrometer by application of sustained off-resonance irradiation (SORI) on a selected tetrameric charge state. The technique of single-frequency quadrupole excitation was used to accomplish selected-ion accumulation of the 14 + charge state of the tetramer during ion injection. Subsequent low energy SORI combined with broadband quadrupole cooling produced the 7 + monomer and 7 + trimer species, as well as the 6 + monomer and 8 + trimer complementary ions. The observed asymmetric breakup of the tetramer is qualitatively explained by using physical models.     

High resolution millimeter-wave spectroscopy of cyclopropylphosphine-borane

The microwave spectrum of cyclopropylphosphine-borane, C(3)H(5)PH(2)-BH(3), has been investigated in the frequency range 150-195 GHz. The spectral assignment was supported by high level ab initio calculations. Two stable conformations have been predicted: the most stable antiperiplanar form and synclinal form that is higher in energy by 7.3 kJ/mol. In the observed spectra, only the most stable antiperiplanar (ap) form has been assigned. The analysis of the rotational spectra in the lowest excited vibrational states of the ap conformer has enabled determination of the potential function for the C-P torsional mode in the vicinity of equilibrium position. The barrier to internal rotation of the BH(3) top has been determined to be 9.616(15) kJ/mol and agrees well with quantum chemical calculations.     

Isomerical and structural determination of N-hydroxyurea: a matrix isolation and theoretical study

The structure, isomerization pathways and vibrational spectra of the important N-hydroxyurea (HU) molecule were studied by matrix isolation FT-IR spectroscopy and molecular orbital calculations undertaken at the MP2/6-311++G(2d,2p) level of theory. In agreement with theoretical predictions, 1Ea represents the most stable keto isomer in the gas-phase, being the dominant species trapped in argon matrices, while the 1Za isomer also contributes to the spectrum of isolated HU molecules. According to the calculated abundance values at the temperature of evaporation of the compound (393 K), the 1Ea and 1Za isomers together with a small contribution of 1Eb are expected to appear in the experimental spectra. Since the barrier for interconversion 1Ea? 1Eb is only ～2 kJ mol(-1), these two isomers are in equilibrium in the matrices and, at low temperature, the population of the less stable 1Eb form is too small to be observed. Full assignment of the observed spectra of N-hydroxyurea and its deuterium analogue was undertaken on the basis of comparison with theoretical data.     

C60的一些氨基酸衍生物结构、光谱的理论研究

用INDO/2系列方法研究了C60与氨基酸加成产物的结构和UV谱, 表明两种加成产物异构体---[6, 6]闭环和[6, 5]闭环异构体, 均具有Cs对称性, 且[6, 6]闭环异构体具有更低的生成热,因而更稳定, 以优化构型为基础, 计算了化合物的UV谱, 与实验值一致, 同时对电子跃迁进行理论指认, 讨论了产物UV谱带红移的原因。     

The two-photon fluorescence excitation spectrum of dibenzothiophene

The two-photon fluorescence excitation spectra of dibenzothiophene crystal (≈77 K) and solution have been investigated in the spectral region 30000–38000 cm^?1 (≈3350-2600 Å). Two electronic transitions have been found. Both the oriented gas model results applied to the crystal and the circular/linear polarization ratio to the solution claim for an ¹A₁ ← ¹A₁ assignment of the transitions. Not completely resolved vibronic structures in the crystal spectrum were observed for both transitions for which a tentative assignment to A₁ total symmetry is discussed. It has been found that the intensity of the first electronic band system arises not only from a purely electronic (i.e., Franck-Condon) mechanism, but also from its interaction with a higher state through totally symmetric vibrations.     

Study of NH stretching vibrations in small ammonia clusters by infrared spectroscopy in He droplets and ab initio calculations

Infrared spectra of the NH stretching vibrations of (NH3)n clusters (n = 2-4) have been obtained using the helium droplet isolation technique and first principles electronic structure anharmonic calculations. The measured spectra exhibit well-resolved bands, which have been assigned to the nu1, nu3, and 2nu4 modes of the ammonia fragments in the clusters. The formation of a hydrogen bond in ammonia dimers leads to an increase of the infrared intensity by about a factor of 4. In the larger clusters the infrared intensity per hydrogen bond is close to that found in dimers and approaches the value in the NH3 crystal. The intensity of the 2nu4 overtone band in the trimer and tetramer increases by a factor of 10 relative to that in the monomer and dimer, and is comparable to the intensity of the nu1 and nu3 fundamental bands in larger clusters. This indicates the onset of the strong anharmonic coupling of the 2nu4 and nu1 modes in larger clusters. The experimental assignments are compared to the ones obtained from first principles electronic structure anharmonic calculations for the dimer and trimer clusters. The anharmonic calculations were performed at the M?ller-Plesset (MP2) level of electronic structure theory and were based on a second-order perturbative evaluation of rovibrational parameters and their effects on the vibrational spectra and average structures. In general, there is excellent (<20 cm(-1)) agreement between the experimentally measured band origins for the N-H stretching frequencies and the calculated anharmonic vibrational frequencies. However, the calculations were found to overestimate the infrared intensities in clusters by about a factor of 4.