Internal Ring Angles Research Articles

Structural variation, π-charge transfer, and transmission of electronic substituent effects through the carbon-carbon triple bond in β-substituted phenylacetylenes: a quantum chemical study, and a comparison with (E)-β-substituted styrenes

The transmission of electronic substituent effects through a carbon-carbon triple bond has been investigated from the small changes induced by a variable substituent X on the geometry of the phenyl group in β-substituted phenylacetylenes, Ph−C≡C−X. The geometries of many such molecules were determined by quantum chemical calculations at the B3LYP/6-311++G** level of theory. The structural variation of the phenyl probe is best represented by a linear combination of the internal ring angles, termed SFPHA. Multiple regression analysis of SFPHA using appropriate explanatory variables reveals a composite polar effect. The main components are, in order of decreasing importance: (i) the field effect of the substituent, enhanced by field-induced π-polarization of the −C≡C− spacer, (ii) the resonance-induced field effect, and (iii) the electronegativity effect. These results are compared with those obtained for (E)-β-substituted styrenes, Ph−CH=CH−X, using the same statistical technique, applied to a structural parameter termed SFSTY. The comparison shows that in β-substituted phenylacetylenes, the field effect of the substituent plays a more important role, and the resonance effect a less important, than in (E)-β-substituted styrenes. This is a consequence of the greater polarizability of the C≡C bond as compared with the C=C bond. The role played by the electronegativity of the substituent is the same for the two sets of molecules. A peculiar feature of the −C≡C− spacer is that its π-system consists of two separate parts, corresponding to the two π-bonds that concur to the formation of the triple bond. Only the part orthogonal to the plane of the phenyl probe may interact with the probe π-system, making it possible to transfer π-electron density from substituent to probe, or vice versa. The part coplanar with the phenyl probe may exchange π-electron density with the substituent, but not with the probe. Thus, π-interactions between substituent, spacer, and probe depend critically on the electronic structure and conformation of the substituent. We have classified the substituents in three categories according to the different types of π-interactions that occur in β-substituted phenylacetylenes. First category substituents exchange π-electron density with the orthogonal part of the π-system of the spacer, and hence with the π-system of the probe. While the orthogonal part of the π-system of the spacer acts as a π-electron channel, the coplanar part cannot exchange electrons with the π-system of the phenyl probe. With second category substituents, the conformation of the molecule inhibits the exchange of π-electron density between substituent and probe. The substituent can only exchange π-electrons with the part of the π-system of the spacer that is coplanar with the phenyl probe. Third category substituents exchange π-electron density with both parts of the π-system of the spacer. While the π-charge transferred into the orthogonal part can be further delocalized into the π-system of the phenyl probe, the π-charge transferred into the coplanar part cannot and remains blocked in the spacer. From there, however, it gives rise to an inductive effect that polarizes the π-electron system of the phenyl probe. The π-charge distribution in 45 β-substituted phenylacetylenes and the corresponding (E)-β-substituted styrenes has been determined by Natural Atomic Orbital analysis at the B3LYP/6-311++G** level of theory. The results of the analysis support our classification of the substituents in three categories and reveal the existence of a number of important correlations involving charges as well as structural parameters.

Transmission of electronic substituent effects along polyenic chains: a quantum chemical study based on structural variation and π-charge distribution

The transmission of electronic substituent effects along chains of conjugated double bonds has been investigated by analyzing the small structural changes induced by a variable substituent X in the phenyl group of Ph–(CH=CH) n –X molecules (n = 2, 3, and 4). The structures of many such molecules with charged or dipolar substituents have been determined from quantum chemical calculations at the B3LYP/6-311++G** level of theory. The structural variation of the phenyl probe is best represented by a linear combination of the internal ring angles, termed S F PE (n = 2, 3, and 4). Multiple regression analysis of the S F PE parameters using appropriate explanatory variables reveals a composite electronic effect, the main component of which is the field effect of the variable substituent, enhanced by field-induced π-polarization of the polyenic chain. Also important is the role of resonance-induced field effects. An electronegativity term contributes significantly to the structural variation of the phenyl probe in (E)-β-substituted styrenes, Ph–CH=CH–X, but is marginally significant in Ph–(CH=CH)2–X molecules and not significant at all in Ph–(CH=CH)3–X and Ph–(CH=CH)4–X molecules. The structural substituent parameters S F PE2 , S F PE3 , S F PE4 , as well as S F STY from (E)-β-substituted styrenes, are all correlated to each other. However, even though the correlation coefficients are high, it appears unequivocally that the data points corresponding to dipolar substituents and those corresponding to charged groups are aligned along slightly different straight lines. An analysis of π-charge distribution in Ph–(CH=CH) n –X molecules (n = 1–4) has also been carried out. It appears that as the number of double bonds increases, the π-charge transmitted from the variable substituent to the hydrocarbon frame becomes larger, while the π-charge transmitted to the phenyl probe becomes smaller. In each of the three series of Ph–(CH=CH) n –X molecules (n = 2, 3, and 4), the π-charge of the phenyl probe is linked by an excellent nonlinear relationship to the corresponding structural substituent parameter S F PE (n = 2, 3, and 4). The effect of the variable substituent on the geometry of the polyenic chain has been studied by analyzing the alternation of C–C bond lengths along the chain in Ph–(CH=CH)4–X molecules. The alternation is most pronounced and regular when the variable substituent X is an uncharged group, irrespective of whether it is a π-acceptor or a π-donor. For the five strongest resonant substituents in our data set (namely, the charged groups CH2 +, CH2 −(c), NH−, O−, and N2 +), there is a region in the chain where the alternation between adjacent C–C bonds decreases and inverts, a structural feature known as geometric soliton.

Transmission of electronic substituent effects across the 1,12-dicarba-closo-dodecaborane cage: a computational study based on structural variation, atomic charges, and ¹³C NMR chemical shifts.

The ability of the 1,12-dicarba-closo-dodecaborane cage to transmit long-range substituent effects has been investigated by analyzing the structural variation of a phenyl probe bonded to C1, as caused by a remote substituent X at C12. The geometries of 41 Ph-CB10H10C-X molecules, including 11 charged species, have been determined by MO calculations at the B3LYP/6-311++G** level of theory. The structural variation of the phenyl probe is best represented by a linear combination of the internal ring angles, termed SF(CARB). Multiple regression analysis of SF(CARB), using appropriate explanatory variables, reveals the presence of resonance effects, superimposed onto the field effect of the remote substituent. The ability of the para-carborane cage to transmit resonance effects is, on average, about one-half of that of the para-phenylene frame in coplanar para-substituted biphenyls. Analysis of the π-charge variation of the phenyl probe confirms that the para-carborane frame is less capable than the coplanar para-phenylene frame of transmitting π-electrons from the remote substituent to the phenyl probe, or vice versa. The para-carborane cage is a better π-acceptor than π-donor; this makes π-donor substituents less effective than π-acceptors in exchanging π-electrons with the phenyl probe across the cage. When the remote substituent is an uncharged group, the para-carborane cage acts as a very weak π-acceptor toward the phenyl probe. The structural variation of the para-carborane cage has also been investigated. It consists primarily of a change of the C1···C12 nonbonded separation, coupled with a change of the five B-C-B angles at C12. This concerted geometrical change is controlled by the electronegativity of the substituent and the resonance interactions occurring between substituent and cage. These, however, appear to be important only when π-donor substituents are involved. The (13)C NMR chemical shifts of the para-carbon of the phenyl probe correlate nicely with SF(CARB), pointing to the reliability of these quantities as measures of long-range substituent effects. On the contrary, the (11)B and (13)C chemical shifts of the cage atoms do not convey information on electronic substituent effects.

A computational study of 4-substituted biphenyls in their minimum energy conformation: twist angles, structural variation, and substituent effects

The role of electronic substituent effects in determining the twist angle of 4-substituted biphenyls and the mechanism of their propagation through the biphenyl frame have been studied by quantum chemical calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory, carried out on 41 p-Ph–C6H4–X molecules in their equilibrium conformation. The twist angle φ about the C–C bond connecting the two benzene rings, ca 45° in unsubstituted biphenyl, almost invariably decreases when a para substituent is present. In the majority of cases the difference ∆φ = φ(X) − φ(H) is small, less than a couple of degrees; the decrease of φ becomes substantial only when the substituent is a charged π-donor or π-acceptor. Multiple regression analysis using appropriate explanatory variables indicates a quadratic dependence of ∆φ on the π-donor/acceptor ability of the substituent, and, to a lesser extent, on its field effect. The propagation of substituent effects through the biphenyl frame has been studied from the structural variation of the Ph group acting as a probe (ring B). This is best measured by a linear combination of the internal ring angles, termed S F BIPH(e) . Multiple regression analysis of S F BIPH(e) reveals a composite field effect, a substantial proportion of which is originated by resonance- and polarization-induced π-charges on the carbon atoms of the –C6H4– spacer (ring A). Extended conjugation between ring A and ring B, caused by resonance and polarization effects, gives rise to quadratic terms in the regression. The reliability of S F BIPH(e) as a measure of long-range polar effects is supported by correlations with various experimental parameters.

Structural variation, π-charge transfer, and transmission of electronic substituent effects in (E)-β-substituted styrenes: a quantum chemical study

The nature and transmission mechanism of substituent effects in (E)-β-substituted styrenes, C6H5–CH=CH–X, have been investigated from the structural changes induced by a variable substituent on the phenyl group. The molecular structures of 46 (E)-β-substituted styrenes were determined from MO calculations at the B3LYP/6-311++G** level of theory. The structural variation of the phenyl probe is best represented by two orthogonal linear combinations of the internal ring angles, S F STY and S R STY . Regression analysis of S F STY using appropriate explanatory variables reveals a composite field effect, the main component of which originates from the long-range effect of the substituent enhanced by field-induced π-polarization of the vinylene spacer and resonance-induced field effects. The electronegativity of the substituent also plays a role in determining the value of S F STY . Comparison with coplanar 4-substituted biphenyls reveals that the components of the field effect in the two molecular systems are of the same nature (apart from the electronegativity contribution, which is not present in biphenyl derivatives). However, the structural variation of the phenyl probe is more pronounced in (E)-β-substituted styrenes due to the shorter distance between substituent and probe. Analysis of π-charge distribution shows that the aptitude of the substituents to exchange π-electrons with the styrene and coplanar biphenyl frames is nearly the same. Nevertheless, the π-charge variation on the phenyl probe of (E)-β-substituted styrenes is 57 % greater than the corresponding quantity in coplanar 4-substituted biphenyls. Thus, the vinylene spacer is more effective than the phenylene spacer in transmitting π-charges. The S R STY parameter is related to the amount of π-charge transferred from the –CH=CH–X moiety into the π-system of the benzene ring, or vice versa, due to the resonance effect of the variable substituent and, to a lesser extent, to field-induced π-polarization.

Transmission of Electronic Substituent Effects through a Benzene Framework: A Computational Study of 4-Substituted Biphenyls Based on Structural Variation

The transmission of substituent effects through a benzene framework has been studied by a novel approach, based on the structural variation of the Ph group in p-Ph-C(6)H(4)-X molecules. The molecular structures of many 4-substituted biphenyls were determined from MO calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory. The twist angle between the phenyl probe (ring B) and the benzene framework carrying the substituent (ring A) was set at 90° to prevent π-electron transfer from one ring to the other and at 0° to maximize it. The structural variation of the probe is best represented by a linear combination of the internal ring angles, termed S(F)(BIPH(o)) and S(F)(BIPH(c)) for the orthogonal and coplanar conformations of the molecules, respectively. Regression analysis of these parameters using appropriate explanatory variables reveals a composite field effect, a substantial proportion of which is originated by resonance-induced π-charges on the carbon atoms of ring A. Field-induced polarization of the π-system of ring A also contributes to the structural variation of the probe. Thus, the S(F)(BIPH(o)) parameter is very well reproduced by a linear combination of the π-charges on the ortho, meta, and para carbons of ring A, an uncommon example of a quantitative relationship between molecular geometry and electron density distribution. Comparison of S(F)(BIPH(o)) with the gas-phase acidities of para-substituted benzoic acids shows that, while the deprotonating carboxylic probe is more sensitive to π-electron withdrawal than donation, the phenyl probe is equally sensitive to both. While the ability of the orthogonal biphenyl system to exchange π-electrons with the para substituent is the same as that of the benzene ring in Ph-X molecules, an increase by about 18% occurs when the conformation is changed from orthogonal to coplanar. The structural variation of the probe becomes more complicated, however. This is due to π-electron transfer from one ring to the other, which is shown to introduce quadratic terms in the regressions.

Investigations of primary and secondary amine carbamate stability by 1H NMR spectroscopy for post combustion capture of carbon dioxide

Carbamate formation is one of the major chemical reactions that can occur in solution in the capture of CO2 by amine-based solvents, and carbamate formation makes a significant enthalpy contribution to the absorption-desorption of CO2 that occurs in the absorber/stripper columns of the PCC process. Consequently, the formation of carbamates of selected series of primary and secondary amines over the temperature range (288 to 318)K has been investigated by equilibrium 1H NMR studies, and the stability constants (K9) for the equilibrium:RNH2+HCO3-⇄K9RNHCOO-+H2Oare reported. van’t Hoff analyses have resulted in standard molar enthalpies, ΔHmo, and entropies, ΔSmo, of carbamate formation. A ΔHmo-ΔSmo plot generates a linear correlation for carbamate formation (providing a mean standard molar free energy, ΔGmo, for carbamate formation of about −7kJ·mol−1), and this relationship helps provide a guide to the selection of an amine(s) solvent for CO2 capture, in terms of enthalpy considerations. A linear ΔHmo-ΔSmo plot also occurs for carbamate protonation.The formation of the carbamates has been correlated with systematic changes in composition and structure, and steric effects have been identified by comparing molecular geometries obtained using density functional B3LYP/6-311++G(d,p) calculations. Trends in steric effects have been identified in the series of compounds monoethanolamine (MEA), 1-amino-2-propanol, 2-amino-1-propanol (AP) and 2-amino-2-methyl-1-propanol (AMP). In the case of 2-piperidinemethanol, 2-piperidineethanol and 3-piperidinemethanol, strong intramolecular hydrogen bonding is shown to be the likely cause for lack of carbamate formation, and in the ring systems of pyrrolidine, morpholine, piperidine and thiomorpholine trends in carbamate formation (as given by K9) have been correlated with the internal ring angle at the amine nitrogen, as well as the planarity of the environment around the nitrogen atom.

Molecular structure of p-diisocyanobenzene from gas-phase electron diffraction and theoretical calculations and effects of intermolecular interactions in the crystal on the benzene ring geometry

In this study, the molecular structure of p-diisocyanobenzene has been determined by gas-phase electron diffraction and quantum chemical calculations. The electron diffraction intensities from a previous study by Colapietro et al. (J Mol Struct 125:19–32, 1984) have been reanalyzed using geometrical constraints and initial values of vibrational amplitudes from computations. The equilibrium structure of the molecule has D 2h symmetry, whereas the average geometry in the gaseous phase is best described by a non-planar model of C 2v symmetry. The lowering of symmetry is due to large-amplitude motion of the substituents out of the plane of the benzene ring. The non-planar model has an internal ring angle at the ipso position, ∠aC2–C1–C6 = 120.6 ± 0.2°, about 1° smaller than that from the previous study, but consistent with the quantum chemical calculations. The mean length of the ring C–C bonds and the length of the triple bond are accurately determined as 〈r g(C–C)〉 = 1.398 ± 0.003 A and r g(N≡C) = 1.177 ± 0.002 A, respectively. Comparison with the gaseous isoelectronic molecules p-diethynylbenzene and p-dicyanobenzene shows that the differences in the mean lengths of the ring C–C bonds and in the lengths of the triple bonds determined by electron diffraction are equal or closely similar to the corresponding differences from quantum chemical calculations. The present experimental value of the ipso angle in free p-diisocyanobenzene is slightly, but significantly smaller than that obtained by X-ray crystallography. The difference is confirmed by computational modeling of the crystal structure and appears to be due to –N≡C···H–C intermolecular interactions in the crystal.

Molecular Structure and Benzene Ring Deformation of Three Cyanobenzenes from Gas-Phase Electron Diffraction and Quantum Chemical Calculations

The molecular structures of cyanobenzene, p-dicyanobenzene, and 1,2,4,5-tetracyanobenzene have been accurately determined by gas-phase electron diffraction and ab initio/DFT MO calculations. The equilibrium structures of these molecules are planar, but their average geometries in the gaseous phase are nonplanar because of large-amplitude vibrational motions of the substituents out of the plane of the benzene ring. The use of nonplanar models in electron diffraction analysis is necessary to yield ring angles consistent with the results of MO calculations. The angular deformation of the benzene ring in the three molecules is found to be much smaller than obtained from previous electron diffraction studies, as well as from microwave spectroscopy studies of cyanobenzene. While the deformation of the ring CC bonds and CCC angles in p-dicyanobenzene is well interpreted as arising from the superposition of independent effects from each substituent, considerable deviation from additivity occurs in 1,2,4,5-tetracyanobenzene. The changes in the ring geometry and C ipso-C cyano bond lengths in this molecule indicate an enhanced ability of the cyano group to withdraw pi-electrons from the benzene ring, compared with cyanobenzene and p-dicyanobenzene. In particular, gas-phase electron diffraction and MP2 or B3LYP calculations show a small but consistent increase in the mean length of the ring CC bonds for each cyano group and a further increase in 1,2,4,5-tetracyanobenzene. Comparison with accurate results from X-ray and neutron crystallography indicates that in p-dicyanobenzene the internal ring angle at the place of substitution opens slightly as the molecule is frozen in the crystal. The small geometrical change, about 0.6 degrees , is shown to be real and to originate from intermolecular C identical withN...HC interactions in the solid state.

Electronegativity, Resonance, and Steric Effects and the Structure of Monosubstituted Benzene Rings: An ab Initio MO Study

The deformation of the carbon skeleton of the benzene ring under substituent impact has been analyzed from the structures of 74 monosubstituted derivatives, as determined by ab initio MO calculations. The geometry of the substituted ring is shown to contain valuable information on the electronegativity, resonance, and steric effects of the substituent, and also on other, more subtle effects, affecting primarily the length of the Cipso−Cortho bonds. The results obtained substantially augment previous knowledge from the analysis of experimental geometries (Domenicano, A.; Murray-Rust, P.; Vaciago, A. Acta Crystallogr., Sect. B 1983, 39, 457). Varying the electronegativity of the substituent causes a concerted change of the ring angles at the ipso, ortho, and para positions, coupled with a change in the Cipso−Cortho bond length. The values of the ipso angle span a remarkably wide range, 113−126°. Enhancing the resonance interaction between a substituent and the ring causes a complex pattern of angular distortions, arising from the superposition of two separate effects. The first originates from the decreased length of the C−X bond, and consists primarily in a concerted change of the ipso and ortho angles. It occurs irrespective of whether the substituent is a π donor or a π acceptor. The second effect is associated with π-charge alternation on the ring carbons. It involves all the internal ring angles, and depends on the substituent being a π donor or a π acceptor. These angular changes are generally accompanied by changes in all C−C bond lengths, as expected from an enhanced contribution of polar canonical forms to the electronic structure of the molecule. By using symmetry coordinates, we have derived two orthogonal linear combinations of the internal ring angles, SE and SR, measuring the electronegativity and resonance effects of a substituent, respectively, as seen from their impact on the ring geometry. SE and SR values are affected in a typical way by steric effects.

N1 and N3 linkage isomers of neutral and deprotonated cytosine with trans-[(CH3NH2)2PtII.

A series of complexes obtained from the reaction of trans-[(CH3NH2)2PtII] with unsubstituted cytosine (CH) and its anion (C), respectively, has been prepared and isolated or detected in solution: trans-[Pt(CH3NH2)2(CH-N3)Cl]Cl.H2O (1), trans-[Pt(CH3NH2)2(CH-N3)2](ClO4)2 (1a), trans-[Pt(CH3NH2)2(C-N3)2].2H2O (1b), trans-[Pt(CH3NH2)2(CH-N3)2](ClO4)(2).2DMSO (1c), trans-[Pt(CH3NH2)2(CH-N1)2] (NO3)(2).3H2O (2a), trans-[Pt(CH3NH2)2(C-N1)2].2H2O (2b), trans-[Pt(CH3NH2)2(CH-N1)(CH-N3)](ClO4)2 (3a), trans-[Pt(CH3NH2)2(C-N1)(C-N3)] (3b), and trans-[Pt(CH3NH2)2(N1-CN3)(N3-C-N1)Cu(OH)]ClO(4).1.2H2O (4). X-ray crystal structures of all these compounds, except 3a and 3b, are reported. Complex 2a is of particular interest in that it contains the rarer of the two 2-oxo-4-amino tautomer forms of cytosine, namely that with the N3 position protonated. Since the effect of PtII on the geometry of the nucleobase is minimal, bond lengths and angles of CH in 2a reflect, to a first approximation, those of the free rare tautomer. Compared to the preferred 2-oxo-4-amino tautomer (N1 site protonated) of CH, the rare tautomer in 2a differs particularly in internal ring angles (7-11 sigma). Formation of compounds containing the rare CH tautomers on a preparative scale can be achieved by a detour (reaction of PtII with the cytosine anion, followed by cytosine reprotonation) or by linkage isomerization (N3-->N1) under alkaline reaction conditions. Surprisingly, in water and over a wide pH range, N1 linkage isomers (3a, 2a) form in considerably higher amounts than can be expected on the basis of the tautomer equilibrium. This is particularly true for the pH range in which the cytosine is present as a neutral species and implies that complexation of the minor tautomer is considerably promoted. Deprotonation of the rare CH tautomers in 2a occurs with pKa values of 6.07 +/- 0.18 (1 sigma) and 7.09 +/- 0.11 (1 sigma). This value compares with pKa 9.06 +/- 0.09 (1 sigma) (average of both ligands) in 1a.

S0 and S1 States of Monochlorophenols: Ab Initio CASSCF MO Study

Optimized geometries and total energies for the S0 and S1 states of five monochlorophenols and phenol were calculated using a 10-electron, 8-orbital CASSCF/6-31G(d). We investigated the effects of a substituent chlorine atom and the S1 ← S0 excitation on the geometries and charge distributions. It is common to all monochlorophenols that the substituent chlorine atom makes the C−O bond shorten and that an empirical rule with respect to the internal ring angle concerned with the substituent chlorine atom holds true. The S1 ← S0 excitation enlarges the carbon ring and reduces the C−H, C−O, and C−Cl bond lengths. The O−H bond length and the C−O−H bond angle are almost invariant upon excitation. A structural approach based on van der Waals radii has clarified that cis-o-chlorophenol has a hydrogen bond in the S1 state and that the interaction between the chlorine and oxygen atoms in a trans-o-chlorophenol affects its geometries for the S0 and S1 states. As to the origin energies of the S1 ← S0 transitions, the present method can compete in terms of accuracy with a 8-electron, 7-orbital CASSCF/6-31G(d,p). The increasing order of the calculated origin transition energies of monochlorophenols and phenol is in qualitative agreement with experimental results. Zero-point corrections are important in identifying the electronic spectra of monochlorophenols. The relative stabilities of rotational isomers, and dipole moments as well, have been also studied in relation to experimental results.

(E)-6-methoxy-3-(alpha-methoxybenzylidene)benzo

The title compound, C17H14O4, is an unprecedented new synthetic isoaurone-type enol ether that has the E configuration. The planar furanone ring is fused to a methoxy­benzene ring system, with an interplanar angle of 175.7 (1)°. Due to this ring fusion, the six-membered ring has a significant amount of ring strain, as shown by the internal ring angle range of 115.8 (1)–124.7 (1)°, whereas the vinylic phenyl ring has internal angles between 119.7 (1) and 120.2 (1)°. The mol­ecules form infinite hydrogen-bonding layers along the b direction of the form C—H⋯O, where the keto O atom acts as a bifurcated acceptor. These layers are connected along the c direction by another hydrogen bond with a methoxy H atom as donor. In addition to this connection, the layers are stacked via centres of symmetry by a pair of symmetry-related benzo­furan­one ring systems.

Catena-Poly[[bis(μ-2,6-dimethylpiperidinido-N:N)dilithium]-μ-(N,N,N',N'-tetramethylethylenediamine-N:N')

The title compound, [Li-2(C7H14N)(2)(C6H16N2)](n), is a chain polymer in which strictly planar Li2N2 rings incorporating 2,6-dimethylpiperidinido ligands are linked by N,N,N',N'-tetramethylethylenediamine ligands in a rare bridging role. The Li atom is planar three-coordinate. The internal ring angles are obtuse [107.89(11)degrees] at lithium and acute [72.11(11)degrees] at nitrogen, as is commonly found for lithium amide ring structures.

Molecular structure of phenylsilane: a study by gas-phase electron diffraction and ab initio molecular orbital calculations

The molecular structure of phenylsilane has been determined accurately by gas-phase electron diffraction and ab initio MO calculations at the MP2(f.c.)/6-31G* level. The calculations indicate that the perpendicular conformation of the molecule, with a Si–H bond in a plane orthogonal to the plane of the benzene ring, is the potential energy minimum. The coplanar conformation, with a Si–H bond in the plane of the ring, corresponds to a rotational transition state. However, the difference in energy is very small, 0.13 kJ mol −1, implying free rotation of the substituent at the temperature of the electron diffraction experiment (301 K). Important bond lengths from electron diffraction are: < r g(C–C)>=1.403±0.003 Å, r g(Si–C)=1.870±0.004 Å, and r g(Si–H)=1.497±0.007 Å. The calculations indicate that the C ipso –C ortho bonds are 0.010 Å longer than the other C–C bonds. The internal ring angle at the ipso position is 118.1±0.2° from electron diffraction and 118.0° from calculations. This confirms the more than 40-year old suggestion of a possible angular deformation of the ring in phenylsilane, in an early electron diffraction study by F.A. Keidel, S.H. Bauer, J. Chem. Phys. 25 (1956) 1218.

2,6-Diazido-9-(carboxymethyl)purine Methyl Ester

The title compound (methyl 2,6-diazidopurine-9-acetate, C8H6N10O2) is a potential intermediate for the synthesis of peptidic nucleic acids containing diaminopurine. Its two azido groups are approximately parallel and at their attachment points, the internal ring angles are 1.4 (3)° smaller than in the dichloro homologue.

Effect of intermolecular O-H ⋯ O hydrogen bonding on the molecular structure of phenol: An ab initio molecular orbital study

The effect of intermolecular O-H ⋯ O hydrogen bonding on the molecular structure of phenol has been studied by SCF ab initio MO calculations at the HF/6-31G * level. The systems investigated are eight phenol-water complexes and the dimer and trimer of phenol. Optimized geometries show that hydrogen bond formation causes a consistent pattern of changes in the structure of the molecule. When phenol acts as a proton donor, the expected increase ofr (O-H) is accompanied by a slight decrease ofr(C-O) and of the internal ring angles at theipso andpara positions, and by an increase ofr(Cipso©ortho). These changes suggest that the relative contribution of polar canonical forms to the electronic structure of the molecule increases upon hydrogen bond formation, since this enhances the strength of the interaction. The opposite changes occur when phenol acts as a proton acceptor, except forr(O-H), which is the same as in the free molecule. If phenol acts as a proton donorand as a proton acceptor, the two hydrogen bonds become stronger due to a synergic effect. In this case, however, the structural deformation of the molecule is less pronounced than in the previous cases, due to the opposite effect of the two hydrogen bonds. The available experimental evidence on gas-crystal structural differences for phenol is critically reviewed, also in the light of the present results on gas-phase complexes.

Molecular structure and ring distortion of phenol. An electron diffraction study

The molecular structure of gaseous phenol has been determined by electron diffraction. This has led to an accurate measurement of the internal ring angle at the place of substitution, α = 121.6° ± 0.2°, an important geometrical parameter that is hard to determine accurately by microwave spectroscopy or ab initio MO calculations. Comparison with solid state results suggests that the angle α decreases by about 1° in going from the free molecule to the crystal. This may reflect a change in the electronic properties of the OH substituent, caused by intermolecular hydrogen bonding in the solid state.

Organometallic precursors for the formation of GaN by metal-organic chemical vapour deposition: a study of [(CH3)2GaNH2

The reaction of Ga(CH3)3 with an excess of ammonia leads to the formation of (CH3)3Ga·NH3, which, when heated at 120 °C under 450 Torr nitrogen, decomposes to yield CH4 and the trimer [(CH3)2GaNH2]31 and not, as previously stated, a dimeric species. A single-crystal X-ray study of 1 revealed that it is a molecular species containing a (GaN)3 ring [Ga–N 1.93(2)–2.05(2)Å] in which four consecutive ring atoms are coplanar. The remaining two atoms of the ring are on the same side of the plane so the ring has the sofa conformation. The internal ring angles range from 93.8(8) to 101.3(8)°(for N–Ga–N) and 119.5(9) to 125.3(9)°(for Ga–N–Ga). Compound 1 is monoclinic, space group P21/c, Z= 4. 985 Independent reflections have been measured on a diffractometer and the structure refined to R= 0.076. Spectroscopic evidence shows that 1 retains its trimeric nature in the gas phase. Thus, in the mass spectrum the parent ion is seen, while infrared spectra of mulls of solid 1, of gaseous 1 condensed onto CsI windows and of matrix-isolated 1 are in agreement with the same species being in both solid and gas phases.

ChemInform Abstract: The Crystal Structure of 1,1‐Cyclobutanedicarboxylic Acid at 20 K

The structure is disordered at room temperature, with atom C3 (Cγ) of the cyclobutane ring occupying sites on each side of the plane of the other three ring atoms. At 20 K, the structure is ordered. The space group is P21/c with Z=4; the unit cell dimensions at 20 K are a=5.983(2) A, b=10.374(4) A, c=10.917(3) A, β=101.19(3)°, V=664.8(4) A3. The average Cα–Cβ bond length, 1.560 A, is longer than the average Cβ–Cγ bond length, 1.548 A, and the internal ring angles are all acute, with that at Cα, the smallest, 88.52°, those at Cβ intermediate, average 89.11°, and that at Cγ, the largest 89.35°. The cyclobutane ring deviates only slightly from mm symmetry, with Cγ displaced from the CαCβ plane by 0.40 A. The average C=O and C–OH bond lengths in the carboxylate groups are 1.223 and 1.323 A, with each acid group hydrogen bonded to another acid group through an inversion center (O⋅⋅⋅O contacts, 2.65 A, on average). Examination of subtle differences in structural parameters suggests that substituent effects on c...