Heavy Atom Skeleton Research Articles

Adaptive Reaktion bei der Lösung von flexiblen Molekülen: Oligohydrate von 4‐Hydroxy‐2‐butanon

AbstractDurch Wasser hervorgerufene Strukturveränderungen spielen eine zentrale Rolle in der Chemie und Biologie, sind aber auf molekularer Ebene nach wie vor schwer vorherzusagen, zu messen und zu kontrollieren. Hier untersuchen wir in der Gasphase die größenabhängige Wasseraggregation am flexiblen Molekül 4‐Hydroxy‐2‐butanon und modellieren die Konformationsanpassungsfähigkeit flexibler Substrate an Wirtswassergerüste, sowie die Präferenzen für sequentielles Tröpfchenwachstum. Das Experiment wurde mittels breitbandiger Rotationsspektroskopie durchgeführt und mit quantenchemischen Berechnungen nachvollzogen. Experimentell wurden vom Di‐ bis zum Pentahydrat (4‐Hydroxy‐2‐butanon‐(H2O)n=2–5) zwei verschiedene Isomere beobachtet, einschließlich der 18O‐Isotopologen für die Di‐ und Trihydrate. Um Wassermoleküle effektiv unterzubringen, verändert sich interessanterweise das schwere Atomgerüst von 4‐Hydroxy‐2‐butanon in jedem beobachteten Isomer und entspricht nicht der stabilen Konformation des freien Monomers. Alle Solvate gehen von der Alkoholgruppe (Protonendonor) aus, behalten aber die Carbonylgruppe als sekundären Bindungspunkt bei. Die Wassergerüste ähneln stark denen in reinen Wasserclustern und balancieren zwischen der Fähigkeit von 4‐Hydroxy‐2‐butanon, die Orientierung und Position der Wassermoleküle zu steuern, und der Fähigkeit von Wasser, die Konformation des Monomers zu modulieren. Die vorliegende Arbeit liefert somit eine genaue molekulare Beschreibung, wie sich torsionsflexible Moleküle im Verlauf der fortschreitenden Solvatation dynamisch an Wasser anpassen.

Adaptive Response to Solvation in Flexible Molecules: Oligo Hydrates of 4-Hydroxy-2-butanone.

Structural changes induced by water play a pivotal role in chemistry and biology but remain challenging to predict, measure, and control at molecular level. Here we explore size-governed gas-phase water aggregation in the flexible molecule 4-hydroxy-2-butanone, modeling the conformational adaptability of flexible substrates to host water scaffolds and the preference for sequential droplet growth. The experiment was conducted using broadband rotational spectroscopy, rationalized with quantum chemical calculations. Two different isomers were observed experimentally from the di- to the pentahydrates (4-hydroxy-2-butanone-(H2O)n=2-5), including the 18O isotopologues for the di- and trihydrates. Interestingly, to accommodate water molecules effectively, the heavy atom skeleton of 4-hydroxy-2-butanone reshapes in every observed isomer and does not correspond to the stable conformer of the free monomer. All solvates initiate from the alcohol group (proton donor) but retain the carbonyl group as secondary binding point. The water scaffolds closely resemble those found in the pure water clusters, balancing between the capability of 4-hydroxy-2-butanone for steering the orientation and position of the water molecules and the ability of water to modulate the monomer's conformation. The present work thus provides an accurate molecular description on how torsionally flexible molecules dynamically adapt to water along progressing solvation.

Structure and Spectroscopic Insights for CH3PCO Isomers: A High-Level Quantum Chemical Study.

The exploration of phosphorus-bearing species stands as a prolific field in current astrochemical research, particularly within the context of prebiotic chemistry. Herein, we have employed high-level quantum chemistry methodologies to predict the structure and spectroscopic properties of isomers composed of a methyl group and three P, C, and O atoms. We have computed relative and dissociation energies, as well as rotational, rovibrational, and torsional parameters using the B2PLYPD3 functional and the explicitly correlated coupled cluster CCSD(T)-F12b method. Based upon our study, all the isomers exhibit a bent heavy atom skeleton with CH3PCO being the most stable structure, regardless of the level theory employed. Following in energy, we found four high-energy isomers, namely, CH3OCP, CH3CPO, CH3COP, and CH3OPC. The computed adiabatic dissociation energies support the stability of all [CH3, P, C, O] isomers against fragmentation into CH3 and [P, C, O]. Torsional barrier heights associated with the methyl internal rotation for each structure have been computed to evaluate the occurrence of possible A-E splittings in the rotational spectra. For the most stable isomer, CH3PCO, we found a V3 barrier of 82 cm-1, which is slightly larger than that obtained experimentally for the N-counterpart, CH3NCO, yet still very low. Therefore, the analysis of its rotational spectrum can be anticipated as a challenging task owing to the effect of the CH3 internal rotation. The complete set of spectroscopic constants and transition frequencies reported here for the most stable isomer, CH3PCO, is intended to facilitate eventual laboratory searches.

Laboratory rotational spectroscopy and astronomical search for 2-ethylacrolein

The rotational spectrum of 2-ethylacrolein was investigated by pulsed jet Fourier transform microwave spectroscopy complementary with quantum chemical calculations. The rotational spectra of both conformers cis and gauche and their all mono-substituted 13C and 18O isotopologues were measured and assigned in natural abundance, allowing the accurate structural determinations on their heavy-atom skeletons. The relative population ratio of cis to gauche in pulsed jets was estimated to be ≈ 3:1. With the accurate rotational transition frequencies, radio astronomical searches toward the massive star-forming region Sgr B2(N) were carried out by using the Shanghai Tianma 65 m radio telescope. No transition lines belonging to 2-ethylacrolein were detected, the upper limits of column densities for cis and gauche were derived to be 7.52 × 1013 cm−2 and 1.29 × 1015 cm−2, respectively.

On the planarity of benzyl cyanide

The microwave spectra of benzyl cyanide, C6H5CH2CN, were recorded using a molecular jet Fourier transform microwave spectrometer operating in the frequency range 2–26.5 GHz. Analyses of the 14N quadrupole hyperfine structures were performed simultaneously with the rotational frequencies. Rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants χaa and χbb − χcc were all determined with very high accuracy in a fit containing 465 lines with standard deviation within the measurement accuracy. Results from quantum chemical calculations at many levels of theory employing the MP2 method indicate a non-planar heavy atom skeleton structure, where the CH2CN group is tilted up to 50° with respect to the phenyl ring. This conflicts with results from many calculations employing density functional theory methods and also with the conclusion of a previous microwave spectroscopic work (X.-Z. Liu, R.K. Bohn, S.A. Sorenson, N.S. True, J. Mol. Struct.243 (1991) 325–339), stating that the CH2CN group and the phenyl ring are in-plane. By comparing the planar defect with those of several planar molecules reported in the literature, and with extensive support from quantum chemistry, we present for the first time evidence suggesting that the heavy atom structure of benzyl cyanide is probably not planar.

Mechanism and kinetics of the oxidation of 1,3-butadien-1-yl (n-C4H5): a theoretical study.

Ab initio CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) calculations of the C4H5O2 potential energy surface have been combined with Rice-Ramsperger-Kassel-Marcus Master Equation (RRKM-ME) calculations of temperature- and pressure-dependent rate constants and product branching ratios to unravel the mechanism and kinetics of the n-C4H5 + O2 reaction. The results indicate that the reaction is fast, with the total rate constant being in the range of 3.4-5.6 × 10-11 cm3 molecule-1 s-1. The main products include 1-oxo-n-butadienyl + O and acrolein + HCO, with their cumulative yield exceeding 90% at temperatures above 1500 K. Two conformers of 1-oxo-n-butadienyl + O are formed via a simple mechanism of O2 addition to the radical site of n-C4H5 followed by the cleavage of the O-O bond proceeding via a van der Waals C4H5OO complex. Alternatively, the pathways leading to acrolein + HCO involve significant reorganization of the heavy-atom skeleton either via formal migration of one O atom to the opposite end of the molecule or its insertion into the C1-C2 bond. Not counting thermal stabilization of the initial peroxy adducts, which prevails at low temperatures and high pressures, all other products share a minor yield of under 5%. Rate constants for the significant reaction channels have been fitted to modified Arrhenius expressions and are proposed for kinetic modeling of the oxidation of aromatic molecules and 1,3-butadiene. As a secondary reaction, n-C4H5 + O2 can be a source for the formation of acrolein observed experimentally in oxidation of the phenyl radical at low combustion temperatures, whereas another significant (secondary) product of the C6H5 + O2 reaction, furan, could be formed through unimolecular decomposition of 1-oxo-n-butadienyl. Both the n-C4H5 + O2 reaction and unimolecular decomposition of its 1-oxo-n-butadienyl primary product are shown not to be a substantial source of ketene.

2-Propionylthiophene: planar, or not planar, that is the question.

The long-standing ambiguity of the molecular planarity when an alkyl group is attached to a system with conjugated double bonds is a great challenge for both experiments and theory. This also holds true for the case of 2-propionylthiophene (2PT) where a propionyl group is attached at the second position of the planar, aromatic thiophene ring. Results from quantum chemistry at the MP2 level of theory, showing that in the two conformers syn- and anti-2PT the ethyl group of the propionyl moiety is slightly tilted out of the thiophene ring plane, conflict with those from the other methods, stating that the ethyl group is in-plane with the thiophene ring. In the microwave spectrum, both syn- and anti-2PT were observed, and their geometry parameters such as the rotational and quartic centrifugal distortion constants were precisely determined. The experimental heavy atom skeleton obtained by isotopic substitutions revealed a tiny, but non-zero tilt angle of the ethyl group out of the thiophene plane, thereby convincingly confirming the non-planarity of 2-propionylthiophene. This conclusion was further supported by the inertial defects calculated from the experimental rotational constants. Finally, splittings arising from the internal rotation of the terminal methyl group were analysed, yielding torsional barriers of 806.94(54) cm-1 and 864.5(88) cm-1 for the two observed conformers, respectively.

Intrinsic Optical Activity and Large-Amplitude Displacement: Conformational Flexibility in (R)-Glycidyl Methyl Ether.

The dispersive optical activity of (R)-(-)-glycidyl methyl ether (R-GME) has been interrogated under ambient vapor-phase and solution-phase conditions, with quantum-chemical analyses built on density functional (B3LYP and CAM-B3LYP) and coupled-cluster (CCSD) implementations of linear-response theory exploited to interpret experimental findings. Inherent flexibility of the heavy atom skeleton leads to nine low-lying structural isomers that possess distinct chiroptical and physicochemical properties, as evinced by marked changes in the magnitude and the sign of rotatory powers observed in various media. These species are interconverted by independent motion along two large-amplitude torsional coordinates and are stabilized differentially by interaction with the surroundings, thereby reapportioning their relative contributions to the collective response evoked from a thermally equilibrated ensemble. The intrinsic behavior exhibited by isolated (vapor-phase) R-GME molecules was calculated through use of both conformer-averaging and restricted vibrational-averaging procedures, the former affording moderately good agreement with measurements of optical rotatory dispersion (ORD) and the latter providing strong evidence for sizable effects arising from vibrational degrees of freedom. A similar conformer-averaging ansatz based on the polarizable-continuum model (PCM) for implicit solvation was deployed to elucidate R-GME specific-rotation parameters acquired for six dilute solutions. This approach gave reasonable predictions for sodium D-line (589.3 nm) experiments performed in the extremes of solvent polarity represented by cyclohexane and acetonitrile but failed to reproduce the overall shape of ORD profiles and suggested more complex processes might be involved in the case of an aromatic medium.

Molecular structure, infrared spectra, photochemistry, and thermal properties of 1-methylhydantoin.

The structural, vibrational, and photochemical study of 1-methylhydantoin (1-MH, C4H6N2O2) was undertaken by matrix isolation infrared spectroscopy (in argon matrix; 10 K), complemented by quantum chemical calculations performed at the DFT(B3LYP)/6-311++G(d,p) level of approximation. The theoretical calculations yielded the Cs symmetry structure, with planar heavy atom skeleton, as the minimum energy structure on the potential energy surface of the molecule. The electronic structure of this minimum energy structure of 1-MH was then studied in detail by means of the natural bond orbital (NBO) and atoms in molecules (AIM) approaches, allowing for the elucidation of specific characteristics of the molecule's σ and π electronic systems. The infrared spectrum of the matrix-isolated 1-MH was fully assigned, also with the help of the theoretically predicted spectrum of the compound, and its UV-induced unimolecular photochemistry (λ ≥ 230 nm) was investigated. The compound was found to fragment to CO, isocyanic acid, methylenimine, and N-methyl-methylenimine. Finally, a thermal behavior investigation on 1-MH samples was carried out using infrared spectroscopy (10 K until melting), differential scanning calorimetry and polarized light thermal microscopy. A new polymorph of 1-MH was identified. The IR spectra of the different observed phases were recorded and interpreted.

Quantum delocalization of benzene in the ring puckering coordinates

The effect of quantum mechanical delocalization of atomic nuclei on the conformation of the six-membered ring structure in two hydrocarbons, cyclohexane and benzene, is investigated using ab initio path integral approach. A striking feature of benzene species is revealed using ring puckering coordinate representation, which demonstrates that the zero point motion of the heavy atom skeleton dominates over the out-of-plane thermal motions of the ring. Even more unexpected is the fact, that this is true not only at low temperature of 150 K, at which such behavior would not be surprising, but also at room temperature, where the nuclear quantum effects are usually of lesser importance, especially in the case of such heavy nuclei as carbon. In view of this finding the planar conformation of benzene, whose equilibrium (T = 0 K) geometry results from the well-known properties of the electronic structure, can be elucidated also at nonzero temperature. According to our simulations, it appears as a consequence of quantum delocalization of the carbon nuclei rather than a trivial time average over the classical configurations of the puckered ring. This interesting behavior is contrasted with the clearly nonplanar structure of cyclohexane, whose ring puckering states can be unequivocally assigned even if the nuclear delocalization is taken into account. © 2014 Wiley Periodicals, Inc.

The microwave spectrum of 3,3,3-trifluoropropionic acid

The rotational spectrum of 3,3,3-trifluoropropionic acid was measured by supersonic jet chirped-pulse Fourier transform microwave spectroscopy in the frequency range from 8 to 18GHz. The rotational constants A=3965.05187(53) MHz, B=1343.14600(28) MHz and C=1228.90710(29) MHz and three quartic centrifugal distortion constants have been determined for the first time. The analysis of the spectrum indicates that the observed conformer is the trans form of the acid with a planar heavy atom skeleton. The results are compared with the supporting ab initio calculations for this molecule.

Ab initio MCSCF study on several azide molecules: energy component analysis of the pseudo-Jahn–Teller effect

An exploration of the most stable geometrical structures for the azide molecules R–N3 (R = H, BeH, BH2, CH3, C6H5) has been carried out by using the ab initio multi-configuration self-consistent field (MCSCF) method together with the aug-cc-pVTZ basis sets. The unknown hydridoberyllium azide (R = BeH) is predicted to suffer no pseudo-Jahn–Teller (JT) distortion and, hence, to assume a linear structure of C∞v symmetry. In contrast, each of the others is found to adopt a trans bent Cs structure with regard to an interior NN bond. To elucidate the nature of the pseudo-JT distortions, energy component analyses have been carried out at all of the stationary points for the relevant molecules. The results show that pseudo-JT stabilizations are classified into two groups, one in which the stability of the bent structure about the azido group arises from lowering of the interelectronic and internuclear repulsion energies and the other in which the stability results from lowering of the electron-nuclear attraction energy. These energy behaviors are accounted for in terms of an elongation or a contraction of the heavy-atom skeleton and a charge migration from some part of the molecule to the other ones. Moreover, it is shown that the theoretical structural characteristics for the known molecules are in good agreement with available experimental facts.

Equilibrium Structures of Heterocyclic Molecules with Large Principal Axis Rotations upon Isotopic Substitution

Equilibrium structures, r(e), of the heterocyclic molecules oxirane, furazan, furan, ethylene ozonide, and 1,3,4-oxadiazole have been determined using three different, somewhat complementary techniques: a completely experimental technique (r(m)), a semiexperimental technique (r(e)(SE), whereby equilibrium rotational constants are derived from experimental effective ground-state rotational constants and corrections based principally on an ab initio cubic force field), and an ab initio technique (r(e)(BO), whereby geometry optimizations are usually performed at the coupled cluster level of theory including single and double excitations augmented by a perturbational estimate of the effects of connected triple excitations [CCSD(T)] using quadruple-ζ Gaussian basis sets). All these molecules are asymmetric tops with the moment of inertia I(c) much larger than the other two moments of inertia, I(a) and I(b). Molecules of this shape experience a large rotation of the principal axis system upon certain isotopic substitutions. For such isotopologues it is difficult to obtain a good structural fit to the semiexperimental moments of inertia I(a) and I(b), which may significantly reduce the accuracy of the r(e)(SE) structural parameters. The origin of this difficulty is explained. For the heavy-atom skeleton of these molecules it was possible to determine a rather accurate empirical mass-dependent structure without a priori knowledge of the equilibrium structure.

ChemInform Abstract: The Molecular Structures of Gaseous Tetrakis(dimethylamino)diboron, B2( NMe2)4, and Tetrakis(methoxy)diboron, B2(OMe)4, as Determined by Electron Diffraction.

The structures of gaseous B2(NMe2)4 and B2(OMe)4 have been determined by electron diffraction. The results indicate that the amine adopts the expected fully staggered conformation with D2 symmetry whilst, surprisingly, the methoxy derivative has not a planar heavy-atom skeleton as thought previously, but a partially staggered conformation. Salient structural parameters (ra) are (i) for B2(NMe2)4, r(B–B) 176.2(1.1) and r(B–N) 140.8(3) pm; NBBN 90.0(1.1), NBN 124.0(5) and BBNC 19.7(1.1)°; and (ii) for B2(OMe)4, r(B–B) 172.0(6) and r(B–O) 136.9(3) pm; OBBO 49.5(1.2), OBO 119.9(4) and BBOC 12.1(2.7) and 21.9(1.9)°(clockwise and anticlockwise out of the BBO plane).

Molecular Structure of 9H-Adenine Tautomer: Gas-Phase Electron Diffraction and Quantum-Chemical Studies

Molecular geometry of 9H-adenine tautomer was calculated by MP2 method using several basis sets (up to cc-pVQZ). According to the results of all quantum-chemical calculations, the molecule has an essentially planar heavy-atom skeleton and a quasi-planar amino group. Since the bond lengths of adenine are of similar magnitude, the structural problem could not be solved by the gas-phase electron diffraction (GED) method alone. Therefore the differences between similar bond lengths derived from ab initio geometry and rotational constants from microwave (MW) spectroscopic study (Brown, R. D.; et al. Chem. Phys. Lett. 1989, 156, 61) were used as supplementary data. To bring the data of the different experimental methods to the same basis (equilibrium structure), GED internuclear distances r(a) and MW rotational constants B(0)((i)) (i = A, B, C) were corrected for vibrational effects. Harmonic and anharmonic corrections were estimated using quadratic and cubic force constants from MP2/cc-pVTZ calculations. Anharmonic corrections to r(a) distances were calculated using improved theoretical approximation. The molecular structure of 9H-adenine is determined experimentally for the first time. Since the GED intensities are not sensitive to hydrogen positions, and small deviations of skeleton cannot be determined with appropriate uncertainty, the molecular configuration of adenine was assumed to be planar (C(s) symmetry) in the GED analysis. The main equilibrium structural parameters determined from GED data supplemented by rotational constants and results of MP2/cc-pVTZ calculations are the following (bond lengths in angstroms and bond angles in degrees with 3sigma in parentheses): r(e)(C2-N1) = 1.344(3), r(e)(C2-N3) = 1.330(3), r(e)(C4-N3) = 1.333(3), r(e)(C4-C5) = 1.401(3), r(e)(C5-C6) = 1.409(3), r(e)(C6-N1) = 1.332(3), r(e)(C5-N7) = 1.380(4), r(e)(C8-N7) = 1.319(3), r(e)(C8-N9) = 1.371(4), r(e)(C4-N9) = 1.377(4), r(e)(C6-N10) = 1.357(4), angle(e)(N1-C2-N3) = 129.0(1), angle(e)(C2-N3-C4) = 111.0(1), angle(e)(N3-C4-C5) = 127.2(1), angle(e)(C4-C5-N7) = 111.9(2), angle(e)(C5-N7-C8) = 103.4(2), and angle(e)(C5-C6-N10) = 121.9(2). The determined experimental bond lengths of adenine are in good agreement with those from MP2 calculations and with experimental bond lengths of pyrimidine and 1H-imidazole (except for the C-C double bond in imidazole). Being close to typical aromatic internuclear distances, the obtained C-C and C-N bond lengths indicate the aromatic nature of this molecule. The calculated aromaticity indexes (GIAO-MP2/cc-pVTZ) confirm this statement.

Raman and infrared spectra, conformational stability, ab initio calculations and vibrational assignment of dimethylsilylisocyanate

AbstractThe Raman (3200‐30 cm−1) and/or infrared spectra (3500 to 400 cm−1) of gaseous, liquid and solid dimethylsilylisocyanate, (CH3)2 Si(H)NCO, have been recorded. The MP2(full) calculations, employing a variety of basis sets with and without diffusion functions, have been used to predict the structural parameters, conformational stability, vibrational fundamental wavenumbers, Raman activities, depolarization values and infrared intensities to support the vibrational assignment. The low wavenumber Raman spectrum of the gas with a significant number of Q‐branches for the SiNC(O) bend is consistent with an essentially linear SiNCO moiety. The ab initio calculations supported this conclusion as all possible orientations of the NCO moiety lead to nearly the same energy. This result is at variance with the conclusion from the electron diffraction study that the heavy atom skeleton was bent with an angle of 152(5)° with one stable cis conformer. It is believed that this reported angle difference from 180° is due to the shrinkage effect. The SiH distance of 1.486 Å has been obtained from the isolated SiH stretching wavenumber. From the adjustment of the ab initio MP2(full)/6‐311+G(d,p) predicted structural parameters, a proposed structure is reported, which is expected to give rotational constants within a few megahertz of the actual ones. These experimental and theoretical results are compared with the corresponding quantities of similar molecules. Copyright © 2009 John Wiley & Sons, Ltd.

Fourier transform microwave spectrum of CO-dimethyl ether

Two sets of 32 rotational transitions were observed for the carbon monoxide-dimethyl ether (CO-DME) complex and two sets of 30 transitions for both (13)CO-DME and C(18)O-DME, in the frequency region from 3.5 to 25.2 GHz, with J ranging from 1<--0 up to 7<--6, by using a Fourier transform microwave spectrometer. The splittings between the two sets of the same transition varied from 2 to 15 MHz, and the two components were assigned to the two lowest states of the internal rotation of CO with respect to DME governed by a twofold potential. A preliminary analysis carried out separately for the two sets of the observed transition frequencies by using an ordinary asymmetric-rotor Hamiltonian indicated that the heavy-atom skeleton of the complex was essentially planar, as evidenced by the "pseudoinertial defects," i.e., the inertial defects, which involve the contributions of the out-of-plane hydrogens of the two methyl groups, I(cc)-I(aa)-I(bb) of -5.764(23) and -5.753(16) uA(2) for the symmetric and antisymmetric states, respectively. All of the observed transition frequencies were subsequently analyzed simultaneously, by using a phenomenological Hamiltonian which was described in a previous paper on Ar-DME and Ne-DME [Morita et al., J. Chem. Phys. 124, 094301 (2006)]. The rotational constants thus derived were analyzed to give the distance between the centers of gravity of the two component molecules, DME and CO, to be 3.682 A and the angle between the CO and the a-inertial axes to be 75.7 degrees ; the C end of the CO being closer to the DME. Most a-type transitions were observed as closely spaced triplets, which were ascribed to the internal rotation of the two methyl tops of DME. The V(3) potential barrier was obtained to be 772(2) cm(-1) from the first-order Coriolis coupling term between the internal rotation and overall rotation, which is about 82% of V(3) for the DME monomer, whereas the second-order contribution of the coupling to the B rotational constant led to V(3) of 705(3) cm(-1). By assuming a Lennard-Jones-type potential, the dissociation energy was estimated to be E(B)=1.6 kJ mol(-1), to be compared with 1.0 and 2.5 kJ mol(-1) for Ne-DME and Ar-DME, respectively.

Anisotropic displacement parameters for H atoms using an ONIOM approach

X-ray diffraction data cannot provide anisotropic displacement parameters (ADPs) for H atoms, a major outstanding problem in charge-density analysis of molecular crystals. Although neutron diffraction experiments are the preferred source of this information, for a variety of reasons they are possible only for a minority of materials of interest. To date, approximate procedures combine rigid-body analysis of the molecular heavy-atom skeleton, based on ADPs derived from the X-ray data, with estimates of internal motion provided by spectroscopic data, analyses of neutron diffraction data on related compounds, or ab initio calculations on isolated molecules. Building on these efforts, an improved methodology is presented, incorporating information on internal vibrational motion from ab initio cluster calculations using the ONIOM approach implemented in GAUSSIAN03. The method is tested by comparing model H-atom ADPs with reference values, largely from neutron diffraction experiments, for a variety of molecular crystals: benzene, 1-methyluracil, alpha-glycine, xylitol and 2-methyl-4-nitroaniline. The results are impressive and, as the method is based on widely available software, and is in principle widely applicable, it offers considerable promise in future charge-density studies of molecular crystals.

Molecular structure, conformation, potential to internal rotation, and ideal gas thermodynamic properties of 3-fluoroanisole and 3,5-difluoroanisole as studied by gas-phase electron diffraction and quantum chemical calculations

3-Fluoroanisole (3-FA) and 3,5-difluoroanisole (3,5-DFA) have been studied by gas-phase electron diffraction, ab initio (HF and MP2), and density functional theory (B3LYP) calculations. Both molecules have a planar heavy atom skeleton. 3,5-DFA exists as a single conformer of C s symmetry, whereas 3-FA exists as a mixture of two planar conformers of C s symmetry with the syn form (the O–CH 3 bond is oriented toward the fluorine atom) being 0.1–0.2 kcal/mol lower in energy than the anti form (the O–CH 3 bond is oriented away from the fluorine atom). From the experimental scattering intensities the following geometric parameters ( r a distances and ∠ α angles with 3σ uncertainties) were derived for 3,5-DFA: r(C–C) av=1.391(2) Å, r(C Ph–O)=1.359(13) Å, r(C Me–O)=1.427(19) Å, r(C–F) av=1.350(6) Å, ∠C–C–C=116.0–123.6°, ∠C–O–C=118.7(12)°, ∠C2–C1–O=114.9(10)°, ∠C6–C1–O=124.9(10)°, ∠(C–C–F) av=118.4(17)°, and for 3-FA, syn conformer: r(C–C) av=1.393(3) Å, r(C Ph–O)=1.364(13) Å, r(C Me–O)=1.423(14) Å, r(C–F)=1.348(9) Å, ∠C–C–C=117.7–123.1°, ∠C–O–C=118.4(11)°, ∠C2–C1–O=124.7(17)°, ∠C6–C1–O=115.1(17)°, ∠C2–C3–F=118.0(24)°. The mole fractions of the syn and anti conformers were found to be 0.55(17) and 0.45, respectively, in good agreement with the theoretical prediction. Ideal gas thermodynamic functions, S°( T), C p ° ( T ) , H°( T)− H°(0), for anisole, 3-FA, and 3,5-DFA were obtained on the basis of B3LYP calculations. Enthalpies of formations, Δ f H 298 ° , were calculated using a Gaussian-3X (G3X) method and the method of isodesmic reactions. Calculated values of C p ° ( T ) and Δ f H 298 ° for anisole are in good agreement with experimental data.

Synthesis and Characterization of a Novel Bismuth—Molybdenum Oxide and Study of Its Ionic Conducting Behavior.

AbstractFor Abstract see ChemInform Abstract in Full Text.