Interplanar torsion in the S1<--S0 electronic spectrum of jet cooled 1-phenylimidazole

The S(1)<--S(0) transition of 1-phenylimidazole (1PI) has been studied in a supersonic jet expansion by resonant two-photon ionization. The origin band at 36 075 cm(-1) is accompanied by a low frequency progression associated with torsion about the bond connecting phenyl and imidazole groups. Torsional potentials have been determined for both states. In S(0), phi(min)=37.2+/-0.5 degrees and the planar barrier is 339+/-20 cm(-1), while in S(1), phi(min)=17.6+/-0.5 degrees and the planar barrier is 57+/-2 cm(-1). The transition moment alignment is observed to be consistent with an excited state of L(b) character, in spite of the "off-axis" conjugation provided by the imidazole ring. These results are compared with ab initio calculations on both states, performed using Hartree-Fock, M?ller-Plesset second-order perturbation, density functional theory with the Becke3-Lee-Yang-Parr functional, time-dependent density functional theory, configuration interaction singles, and complete active space self-consistent field methods. Solution-phase UV spectra of neutral and protonated 1PI are also reported.     

Impact of neighboring chains on torsional defects in oligothiophenes

Conjugated organic oligomers are central to the development of efficient organic electronic devices and organic photovoltaics. However, the torsional flexibility of many of these organic materials, in particular oligothiophenes, can adversely affect charge transfer properties. Although previous studies have examined the torsional flexibility of oligothiophenes, there have been only limited studies of the effects of interchain interactions on their torsional potentials. B97-D/TZV(2d,2p) was first benchmarked against a CCSD(T)/aug-cc-pVTZ torsional potential for bithiophene as well as SCS-MP2/TZVPP interaction energies for noncovalent sexithiophene (6T) dimers. The effect of neighboring chains on three distinct torsional modes of sexithiophene was studied using B97-D. Complexation with one or more neighboring chains has a dramatic effect on each of these torsional potentials. For example, for two stacked chains, alternated twisting motions are competitive with torsion about a single terminal dihedral angle, and in both cases we predict nonplanar global energy minima and large amplitude torsional motions at room temperature. In other words, the presence of a single neighboring chain induces significant deviations from planarity in oligothiophenes. However, in the environment of crystalline 6T, the trend in predicted torsional potentials match those of isolated chains, but the force constants associated with torsional motions increase by an order of magnitude. Consequently, although individual oligothiophene chains are torsionally flexible and model stacked dimers exhibit extreme deviations from planarity, in crystalline 6T these oligomers are predicted to adopt planar configurations with a steep energetic cost associated with torsional defects.     

2-Nitro-N,N-bis(2-nitro­phenyl­thio)­benzenesulfon­amide

The central N atom of C₁₈H₁₂N₄O₈S is essentially planar, lying 0.071 (2) Å out of the plane defined by the three S atoms. N—S distances are 1.712 (2) and 1.721 (2) Å to S—Ar, and 1.681 (2) Å to SO₂—Ar. The nitro group on the phenyl ring carrying the SO₂ group lies out of plane, with C—C—N—O– torsion angle 69.8 (3)°, while the other two nitro groups are near coplanarity, with torsion angle magnitudes 10.4 (3) and 14.0 (3)°.     

Ab initio study of the torsional potential energy surfaces of N2O3 and N2O4: origin of the torsional barriers

Intrinsic reaction coordinate (IRC) torsional potentials were calculated for N(2)O(4) and N(2)O(3) based on optimized B3LYP/aug-cc-pVDZ geometries of the respective 90 degrees -twisted saddle points. These potentials were refined by obtaining CCSD(T)aug-cc-pVXZ energies [in the complete basis set (CBS) limit] of points along the IRC. A comparison is made between these ab initio potentials and an analytical form based on a two-term cosine expansion in terms of the N-N dihedral angle. The shapes of these two potential curves are in close agreement. The torsional barriers in N(2)O(4) and N(2)O(3) obtained from the CCSD(T)/CBS//B3LYP/aug-cc-pVDZ calculations are 2333 and 1704 cm(-1), respectively. For N(2)O(4) the torsion fundamental frequency from the IRC potential is 87.06 cm(-1), which is in good agreement with the experimentally reported value of 81.73 cm(-1). However, in the case of N(2)O(3) the torsional frequency found from the IRC potential, 144 cm(-1), is considerably larger than the reported experimental values 63-76 cm(-1). Consistent with this discrepancy, the torsional barrier obtained from several different calculations, 1417-1718 cm(-1), is higher than the value of 350 cm(-1) deduced from experimental studies. It is suggested that the assignment of the torsional mode in N(2)O(3) should be reexamined. N(2)O(4) and N(2)O(3) exhibit strong hyperconjugative interactions of in-plane O lone pairs with the central N-N sigma* antibond. Hyperconjugative stabilization is somewhat stronger at the planar geometries because 1,4 interactions of lone pairs on cis O atoms promote delocalization of electrons into the N-N antibond. Calculations therefore suggest that the torsional barriers in these molecules arise principally from a combination of 1,4 interactions and hyperconjugation.     

Theoretical design of a light-driven molecular rotary motor with low energy helical inversion: 9-(5-methyl-2-phenyl-2-cyclopenten-1-ylidene)-9H-fluorene

A light-driven molecular rotary motor of 9-(5-methyl-2-phenyl-2-cyclopenten-1-ylidene)-9H-fluorene (MPCPF) has been designed by means of ab initio complete active space self-consistent field and its second order multireference M?ller-Plesset perturbation methods. In the present model molecule of MPCPF, 9H-fluorene (as a stator) and 5-methyl-2-phenyl-2-cyclopenten-1-ylidene (as a rotor) are directly linked with each other by a C═C double bond. Even by a substitution of phenyl group, MPCPF comes to have a stable P-helical MPCPF and a metastable M-helical MPCPF, and exhibits unidirectionality around the C═C double bond. In addition, interchange of the helicity can proceed with a low energy barrier through a floppy phenyl torsional motion. This is in contrast to previous light-driven molecular rotary motors where the unidirectionality is ensured by rigid and sterically overcrowded rotors. In the full rotary process of MPCPF, therefore, constancy of the rotation speed is expected to be much more improved as well as unidirectionality.     

Biindanylidenes: role of central bond torsion in nonvertical triplet excitation transfer to the stilbenes

The stilbenes were proposed to function as nonvertical triplet excitation (NVET) acceptors for energy-deficient donors because rotation about the central bond diminishes the energy gap between ground and triplet energy surfaces. Recently, the role of central bond torsion in facilitating NVET to cis-stilbene (c-St) was questioned because the behavior of 2,3-diphenylnorbornene as a triplet energy acceptor is similar to that of cis-stilbene. On the basis of the assumption that the rigidity of the norbornene skeleton precludes torsional displacement of the phenyl rings in the triplet state, an alternative mechanism was proposed involving phenyl-vinyl torsion as the key reaction coordinate for NVET to c-St. However, this proposal is inconsistent with theory, which predicts that the triplet state energy minimum corresponds to a geometry with significant displacement of the phenyl rings of 2,3-diphenylnorbornene from a common plane. We now provide experimental evidence demonstrating that central bond torsion is the key coordinate for NVET to stilbenes. Comparison of the activation parameters for the two rigid stilbene analogues, cis- and trans-1,1'-biindanylidene (c-Bi and t-Bi) to those for the stilbenes, shows that the excitation transfer processes remain nonvertical despite the strong structural inhibition of phenyl-vinyl torsion; the relatively small preexponential factors of the respective isomers are almost identical. Their magnitude is a measure of the attenuation introduced by Franck-Condon overlap factors which decrease as the torsional state quantum number corresponding to the transition state increases. These results and results from theoretical calculations are consistent with central bond torsion as the key reaction coordinate in NVET to the biindanylidenes and the stilbenes. The crystal structure of t-Bi shows it to be strictly planar, eliminating phenyl-vinyl torsion toward planarity as a crucial NVET reaction coordinate.     

Biphenyl: A stress tensor and vector‐based perspective explored within the quantum theory of atoms in molecules

We use quantum theory of atoms in molecules (QTAIM) and the stress tensor topological approaches to explain the effects of the torsion φ of the C‐C bond linking the two phenyl rings of the biphenyl molecule on a bond‐by‐bond basis using both a scalar and vector‐based analysis. Using the total local energy density H( r _b), we show the favorable conditions for the formation of the controversial H–H bonding interactions for a planar biphenyl geometry. This bond‐by‐bond QTAIM analysis is found to be agreement with an earlier alternative QTAIM atom‐by‐atom approach that indicated that the H–H bonding interaction provided a locally stabilizing effect that is overwhelmed by the destabilizing role of the C‐C bond. This leads to a global destabilization of the planar biphenyl conformation compared with the twisted global minimum. In addition, the H( r _b) analysis showed that only the central torsional C‐C bond indicated a minimum for a torsion φ value coinciding with that of the conventional global energy minimum. The H–H bonding interactions are found to be topologically unstable for any torsion of the central C‐C bond away from the planar biphenyl geometry. Conversely, we demonstrate that for 0.0° < φ < 39.95° there is a resultant increase in the topological stability of the C nuclei comprising the central torsional C‐C bond. Evidence is found of the effect of the H–H bonding interactions on the torsion φ of the central C‐C bond of the biphenyl molecule in the form of the QTAIM response β of the total electronic charge density ρ( r _b). Using a vector‐based treatment of QTAIM we confirm the presence of the sharing of chemical character between adjacent bonds. In addition, we present a QTAIM interpretation of hyperconjugation and conjugation effects, the former was quantified as larger in agreement with molecular orbital (MO) theory. The stress tensor and the QTAIM H atomic basin path set areas are independently found to be new tools relevant for the incommensurate gas to solid phase transition occurring in biphenyl for a value of the torsion reaction coordinate φ ≈ 5°. © 2015 Wiley Periodicals, Inc.     

Changes in the isotropic shielding of the 17O nucleus upon torsion in terminal oxygen systems: a computational study on their origin

We are presenting a computational study on the isotropic shielding, charge, and orbital contributions to the shielding of oxygen in benzaldehydes (Ar-CHO), nitrobenzenes (Ar-NO2), phenyl isocyanates (Ar-NCO), anilides (Ar-NHCOCH3), and N-sulfinylamines (Ar-NSO). In particular, changes upon ortho substitution of the aromatic ring and upon torsion of the unsubstituted parent molecules are examined. The experimentally observed changes in (17)O chemical shift, be they upfield or downfield, upon substitution by ortho-alkyl groups are reproduced well by the calculations. Relaxed torsional scans of the parent systems reveal that (a) charges change as expected from resonance arguments and (b) changes in isotropic shielding are monotonic and in line with changes upon substitution, with N-sulfinylaniline as an exception. In general, the changes in isotropic shieldings are explained in terms of changes in molecular orbitals, their energies, and relative alignments, whose mixing is magnetically active. Thus, for example, the observed deshielding of (17)O upon methyl substitution and upon torsion of benzaldehyde is mainly caused by a contribution from the pi-type oxygen lone pair, yet how these contributions change is fundamentally different. As a consequence, the experimentally observed downfield shift upon methyl substitution cannot be interpreted to imply a change in torsion angle between the phenyl ring and the aldehyde group. For N-sulfinylaniline, the consecutive downfield shifts upon methyl and tert-butyl substitution and the associated changes in torsion angle are in contrast to the 45 degrees maximum in isotropic shielding that is determined from a relaxed torsional scan.     

Collision-energy-resolved penning ionization electron spectroscopy of phenylacetylene and diphenylacetylene by collision with He*(2(3)s) metastable atoms

Penning ionization of phenylacetylene and diphenylacetylene upon collision with metastable He*(2(3)S) atoms was studied by collision-energy-/electron-energy-resolved two-dimensional Penning ionization electron spectroscopy (2D-PIES). On the basis of the collision energy dependence of partial ionization cross-sections (CEDPICS) obtained from 2D-PIES as well as ab initio molecular orbital calculations for the approach of a metastable atom to the target molecule, anisotropy of interaction between the target molecule and He*(2(3)S) was investigated. For the calculations of interaction potential, a Li(2(2)S) atom was used in place of He*(2(3)S) metastable atom because of its well-known interaction behavior with various targets. The results indicate that attractive potentials localize in the pi regions of the phenyl groups as well as in the pi-conjugated regions of the acetylene group. Although similar attractive interactions were also found by the observation of CEDPICS for ionization of all pi MOs localized at the C[triple bond]C bond, the in-plane regions have repulsive potentials. Rotation of the phenyl groups about the C[triple bond]C bond can be observed for diphenylacetylene because of a low torsion barrier. So the examination of measured PIES was performed taking into consideration the change of ionization energies for conjugated molecular orbitals.     

Anomalous splittings of torsional sublevels induced by the aldehyde inversion motion in the S1 state of acetaldehyde

The G6 group-theoretical high-barrier formalism developed previously for internally rotating and inverting CH3NHD is used to interpret the abnormal torsional splittings in the S1 state of acetaldehyde for levels 14(0-)15(0), 14(0-)15(1), and 14(0-)15(2), where 14(0-) denotes the upper inversion tunneling component of the aldehyde hydrogen and 15 denotes the methyl torsional vibration. This formalism, derived using an extended permutation-inversion group G6m, treats simultaneously methyl torsional tunneling, aldehyde-hydrogen inversion tunneling and overall rotation. Fits to the rotational states of the four pairs of inversion-torsion vibrational levels (14(0+)15(0A,E), 14(0-)15(0A,E)), (14(0+)15(1A,E), 14(0-)15(1A,E)), (14(0+)15(2A,E), 14(0-)15(2A,E)), and (14(0+)15(3A,E), 14(0-)15(3A,E)) are performed, giving root-mean-square deviations of 0.003, 0.004, 0.004, and 0.004 cm(-1), respectively, which are nearly equal to the experimental uncertainty of 0.003 cm(-1). For torsional levels lying near the top of the torsional barrier, this theoretical model, after including higher-order terms, provides satisfactory fits to the experimental data. The partially anomalous K-doublet structure of the S1 state, which deviates from that in a simple torsion-rotation molecule, is fitted using this formalism and is shown to arise from coupling of torsion and rotation motion with the aldehyde-hydrogen inversion.     

Intramolecular charge transfer in 4-aminobenzonitriles does not necessarily need the twist

In electron donor/acceptor species such as 4-(dimethylamino)benzonitrile (DMABN), the excitation to the S(2) state is followed by internal conversion to the locally excited (LE) state. Dual fluorescence then becomes possible from both the LE and the twisted intramolecular charge-transfer (TICT) states. A detailed mechanism for the ICT of DMABN and 4-aminobenzonitrile (ABN) is presented in this work. The two emitting S(1) species are adiabatically linked along the amino torsion reaction coordinate. However, the S(2)/S(1) CT-LE radiationless decay occurs via an extended conical intersection "seam" that runs almost parallel to this torsional coordinate. At the lowest energy point on this conical intersection seam, the amino group is untwisted; however, the seam is accessible for a large range of torsional angles. Thus, the S(1) LE-TICT equilibration and dual fluorescence will be controlled by (a) the S(1) torsional reaction path and (b) the position along the amino group twist coordinate where the S(2)/S(1) CT-LE radiationless decay occurs. For DMABN, population of LE and TICT can occur because the two species have similar stabilities. However, in ABN, the equilibrium lies in favor of LE, as a TICT state was found at much higher energy with a low reaction barrier toward LE. This explains why dual fluorescence cannot be observed in ABN. The S(1)-->S(0) deactivation channel accessible from the LE state was also studied.     

Meta conjugation effect on the torsional motion of aminostilbenes in the photoinduced intramolecular charge-transfer state

The photochemical behavior of a series of trans-3-(N-arylamino)stilbenes (m1, aryl = 4-substituted phenyl with a substituent of cyano (CN), hydrogen (H), methyl (Me), or methoxy (OM)) in both nonpolar and polar solvents is reported and compared to that of the corresponding para isomers (p1CN, p1H, p1Me, and p1OM). The distinct propensity of torsional motion toward a low-lying twisted intramolecular charge-transfer (TICT) state from the planar ICT (PICT) precursor between the meta and para isomers of 1CN and 1Me reveals the intriguing meta conjugation effect and the importance of the reaction kinetics. Whereas the poor charge-redistribution (delocalization) ability through the meta-phenylene bridge accounts for the unfavorable TICT-forming process for m1CN, it is such a property that slows down the decay processes of fluorescence and photoisomerization for m1Me, facilitating the competition of the single-bond torsional reaction. In contrast, the quinoidal character for p1Me in the PICT state kinetically favors both fluorescence and photoisomerization but disfavors the single-bond torsion. The resulting concept of thermodynamically allowed but kinetically inhibited TICT formation could also apply to understanding the other D-A systems, including trans-4-cyano-4'-(N,N-dimethylamino)stilbene (DCS) and 3-(N,N-dimethylamino)benzonitrile (3DMABN).     

Torsional barriers for planar versus twisted singlet styrenes

Kinetic modeling of the temperature-dependent lifetimes of styrene and several methyl-substituted styrenes has been used to obtain the torsional barriers for singlet-state C=C rotation. The barrier for C=C torsion is found to be correlated with the ground-state phenyl-vinyl dihedral angle, varphi, planar styrenes having barriers of approximately 6.5 kcal/mol and moderately twisted styrenes having smaller barriers. Highly twisted styrenes undergo exceptionally rapid intersystem crossing. This unexpected dependence of excited-state behavior on varphi is attributed to a change in the character of the excited states from delocalized for planar styrenes to localized for highly twisted styrenes.     

o-Amino conjugation effect on the photochemistry of trans-aminostilbenes

The excited-state behavior of a series of trans-2-(N-arylamino)stilbenes (aryl = phenyl (o1H), 4-methylphenyl (o1Me), 4-methoxyphenyl (o1OM), and 4-cyanophenyl (o1CN)) and trans-2-(N,N-diphenylamino)stilbene (o2) in both nonpolar and polar solvents is reported and compared to that of the parent trans-2-aminostilbene and the corresponding meta- and para-isomers (m1R and p1R, where R = H, Me, OM, and CN, and m2 and p2). Two types of torsional motions, the D-A torsion that results in a nonfluorescent twisted intramolecular charge transfer (TICT) state and the C═C torsion that leads to the cis isomers, account for the radiationless decays of o1R and o2. The relative efficiencies of these torsions can be readily evaluated from their quantum yields for fluorescence (Φ(f)) and trans → cis isomerization (Φ(tc)). The propensities of the D-A torsion are similar for the ortho and meta isomers, which is 1OM > 1Me and negligible for 1H, 1CN, and 2. The activation parameters determined from temperature-dependent fluorescence lifetimes suggest that the C═C torsion occurs mainly via the triplet state for the ortho systems, a behavior again similar to that of the meta isomers. Whereas the intersystem crossing in o1R, m1R, and m2 is essentially a nonactivated process, it encounters a barrier of 2.7-3.8 kcal mol(-1) in o2. As a result of the barriers that decelerate the radiationless decays and the slow fluorescence rate for o2 in acetonitrile, the observed long fluorescence lifetime 24.5 ns at room temperature reaches a new record for unconstrained trans-stilbenes.     

S0 and S1 state structure, methyl torsional barrier heights, and fast intersystem crossing dynamics of 5-methyl-2-hydroxypyrimidine

We report the analysis of the S1<--S0 rotational band contours of jet-cooled 5-methyl-2-hydroxypyrimidine (5M2HP), the enol form of deoxythymine. Unlike thymine, which exhibits a structureless spectrum, the vibronic spectrum of 5M2HP is well structured, allowing us to determine the rotational constants and the methyl group torsional barriers in the S0 and S1 states. The 0(0)(0), 6a(0)(1), 6b(0)(1), and 14(0)(1) band contours were measured at 900 MHz (0.03 cm(-1)) resolution using mass-specific two-color resonant two-photon ionization (2C-R2PI) spectroscopy. All four bands are polarized perpendicular to the pyrimidine plane (>90% c type), identifying the S1<--S0 excitation of 5M2HP as a 1nπ* transition. All contours exhibit two methyl rotor subbands that arise from the lowest 5-methyl torsional states 0A" and 1E". The S0 and S1 state torsional barriers were extracted from fits to the torsional subbands. The 3-fold barriers are V3" = 13 cm(-1) and V3' = 51 cm(-1); the 6-fold barrier contributions V6" and V6' are in the range of 2-3 cm(-1) and are positive in both states. The changes of A, B, and C rotational constants upon S1 <--S0 excitation were extracted from the contours and reflect an “anti-quinoidal” distortion. The 0(0)(0) contour can only be simulated if a 3 GHz Lorentzian line shape is included, which implies that the S1(1nπ*) lifetime is ~55 ps. For the 6a(0)(1) and 6b(0)(1) bands, the Lorentzian component increases to 5.5 GHz, reflecting a lifetime decrease to ~30 ps. The short lifetimes are consistent with the absence of fluorescence from the 1nπ* state. Combining these measurements with the previous observation of efficient intersystem crossing (ISC) from the S1 state to a long-lived T1 (3nπ*) state that lies ~2200 cm(-1) below [S. Lobsiger, S. et al. Phys. Chem. Chem. Phys. 2010, 12, 5032] implies that the broadening arises from fast intersystem crossing with k(ISC) ≈ 2 × 10(10) s(-1). In comparison to 5-methylpyrimidine, the ISC rate is enhanced by at least 10 000 by the additional hydroxy group in position 2.     

An intrinsic reaction coordinate calculation of the torsion-internal rotation potential of hydrogen peroxide and its isotopomers

Intrinsic reaction coordinate (IRC) calculations of the internal rotation (torsional) potentials for H(2)O(2) and its isotopomers HDO(2) and D(2)O(2) were carried out at the CCSD(T)/CBS//aug-cc-pVDZ level. Two extrapolation methods were used to obtain energies in the complete basis set (CBS) limit. The full IRC potential was constructed from scans from the C(2v) (cis) and C(2h) (trans) transition states to the equilibrium C(2) (gauche) structure. The IRC potential for H(2)O(2) was fit to a five-term Fourier function; coefficients were compared with values obtained from spectroscopic data. The twofold IRC torsional potentials were used to obtain torsional eigenvalues, which yielded values of the transitions between various ntau states. These results compare favorably with Raman and near-infrared data. Our calculations provide values of the cis and trans barriers of 2495 and 364 cm(-1), respectively, which are in good agreement with both previously calculated and experimentally derived values. It appears that coupling between torsional motion and other degrees of freedom is not significant in these molecules.     

Orthogonene redux: a nearly orthogonal alkene predicted to exhibit considerable stability. A computational study

A strongly twisted, as yet unknown, alkene, orthogonene (tetracyclo[8,2,2,0(2,7),0(3,10)]tetradecene-2(3)), was computationally reinvestigated: earlier work had indicated that the stereoisomer with the pair of bridgehead hydrogens on the methine group at one end of the double bond syn to the pair at the other end (C-H/C-H "up-up/up-up", C(2) symmetry, B3LYP/6-31G double bond angle 83 degrees) is at best of low stability, rearranging with a very small barrier to a carbene or possibly a cyclopropane. Here it is shown that the anti ("up-up/down-down") stereoisomer (ideally D(2) symmetry, CASSCF(4,4)/6-31G* double bond torsional dihedral angle 88 degrees) is much more stable: the barrier to rearrangement to a carbene is calculated to be ca. 200 kJ mol(-1) (CASSCF(4,4)/6-31G*), indicating this compound to be a realistic synthetic objective. The vertical ionization energy of this molecule is predicted to be ca. 5.3 eV, comparable to that of the alkali metals and similar to that predicted by others for a hypothetical planar tetracoordinate carbon molecule.     

Single crystal structure of 3-(2-(3-acetylphenyl)hydrazono)pentane-2,4-dione and molecular docking study with CYP450 members for anticancer molecular screening

Coupling of acetyl acetone with diazotized 3-aminoacetophenone was carried out to give the compound 3. The compound was characterized by IR, ¹HNMR, MS and elemental analysis. The X-ray analysis of 3 revels its planar nature with torsion angles between phenyl ring and acetyl group C2–C3–C7–O1 and C2–C3–C7–C8 of 177.8(3)° and ?0.9(4)°, respectively. Small deviations from planarity are evident also by torsion angles N1–N2–C9–C10 and N2–C9–C12–O3 of 176.6(2)° and 2.9(4)°, while the N2–C9–C10–O2 torsion angle with the value of ?165.2(3)° deviates from planarity. Hydrogen-bonded chain formed through C5–H5?O2 is connected with the adjacent antiparallel chain through C11–H11B···π interaction between the methyl group of the acetyl substituent and phenyl ring forming a double-layered chain. N1–N2 shows presence of single bond, showing it to be a hydrazone. Molecular docking study of the title compound with six members from CYP450 family shows encouraging activity.     

Concerning the Conformation of Isolated Benzylideneaniline

From PE.-spectroscopical studies the torsional angle φ of the N-phenyl ring in isolated benzylideneaniline 1 has been found to be definitely smaller than φ= 90°. An approximate valueφ= 36°has been estimated which is even smaller than the one observed in the crystal (φ= 55°) and suggested to prevail also in solutions of 1 . A reevaluation of the gas phase optical spectrum of isolated 1 supports a torsional angle similar to that found in the other phases. Calculations of the most stable conformation of 1 as well as of stilbene and azobenzene by the MINDO/3-technique lead to torsional angles φ= 90° for both phenyl rings in all cases. These results are at variance with the experimental results and suggest that MINDO/3-like its less advanced precursor MINDO/2 or like CNDO/2–is unreliable for low energy processes involving rotation of π-systems connected by essential single bonds. It is concluded that the π-energy of benzylideneaniline, like that of stilbene or azobenzene, would favor a planar conformation. The increased torsional angle in 1 as compared to the other two iso-conjugate systems arises from a larger steric interaction between phenyl- and bridgeprotons.     

Electron diffraction study of the molecular structure and conformation of gaseous 2-furoyl chloride

The molecular structure of 2-furoyl chloride has been investigated by gas-phase electron diffraction at 86°C. Two distinct conformers were identified, a more stable planar form with the furan oxygen and the carbonyl oxygen syn and a less stable planar (or nearly planar) anti form. Assuming that the two forms differ in their geometries only in the O=C---C---O torsion angles and assuming the furan ring to have C_2v symmetry, the results for some of the distances (r_a) and angles (_a) are: r(C---H) = 1.110(20) Å, r(C=O) = 1.207(6) Å, r(C---O) = 1.378(10) Å, r(C??? = 1.465(13) A, (r(C---C)) (average carbon—carbon distance in the furan ring) = 1.392(8) A Δr(C---C) (difference between single and double carbon—carbon distances in the furan ring) = 0.069 A (assumed), r(C---Cl) = 1.787(6) A, C=C---COCl = 131.6(9)°, C=C---O = 110.9(4)°, C=C---H = 127.7(13.4)°, C---C=O = 125.8(8)° and C---C---Cl = 111.8(6)°. At 359 K the observed amount of the conformer with the oxygen atoms syn was 69.8(14.2)%.