Microwave spectrum,dipole moment and structure of isopropyl cyanide

The microwave spectrum of isopropyl cyanide, (CH₃)₂CHCN, has been recorded from 26.5 to 40.0 GHz. Both A- and C-type transitions were observed. The R-branch assignments have been made for the ground and three different excited states. The following structural parameters were obtained: r(C-CN) = 1.501 Å, ∠CCC = 113.8°, and an angle between the CCC plane and the CN bond of 53.8° with reasonable assumptions made for the structural parameters for the isopropyl moiety and the nitrile bond. The dipole moment components were determined to be μ_a = 4.05±0.02, μ_c= 1.4 ± 0.2 and μ_t = 4.29 ±0.10 D. The dipole moment of t-butyl cyanide has been re-measured and found to have a value of4.34±0.04 D. From the relative intensities of the excited state lines, the two torsional modes were found to have frequencies of 200 ±20 and 249 ±10 cm^?1 which gave a periodic barrier to internal rotation of 3.3 kcal mole^?1.     

Microwave spectrum,structure, dipole moment and internal rotation of trans-ethylmethylether

Microwave spectra of ethylmethylether and its eleven isotopically substituted species were measured. The r_s structure of the trans isomer was determined from the observed moments of inertia. Structural parameters of this isomer were roughly equal to those of the reported r_s structure for dimethylether and diethylether. The CH₂-O bond length was definitely shorter by about 0.01 Å than the CH₃-O bond length and the C-C bond length was nearly equal to those of ethylchloride and bromide. The OCH₃ group tilted by about 2° 13' towards lone pair electrons of the oxygen atom while no significant tilt angle was found for the CH₃C group.Dipole moments of the trans isomer for the normal and two deuterated species were determined by Stark-effect measurements. For the normal species, the dipole moment was μ_a = 0.146 ± 0.022 D,μ_b = 1.165 ± 0.020 D and μ_total=1.174 ± 0.022 D making an angle of 7° 5' ± 32' with the b inertial axis. Direction of the dipole moment in the molecule was discussed.From splittings of the observed spectra, barriers to internal rotations of two CH₃ groups were obtained in the one-top approximation. They were 2702 ± 7 and 3300 ± 25 cal mol^?1 for the OCH₃ and CH₃C groups, respectively, from the analysis of splittings in the first excited CH₃ torsional states. The coupling effects among two tops and the skeletal torsion were briefly discussed.     

Very low-pressure pyrolysis (VLPP) of 3,3-dimethylbut-1-yne (tert-butyl acetylene). The heat of formation and stabilization energy of the dimethylpropargyl radical

The unimolecular decomposition of 3,3-dimethylbut-1-yne has been investigated over the temperature range of 933°-1182°K using the technique of very low-pressure pyrolysis (VLPP). The primary process is C? C bond fission yielding the resonance stabilized dimethylpropargyl radical. Application of RRKM theory shows that the experimental unimolecular rate constants are consistent with the high-pressure Arrhenius parameters given by log (k/sec^?1) = (15.8 ± 0.3) - (70.8 ± 1.5)/θ where θ = 2.303RT kcal/mol. The activation energy leads to DH⁰[(CH₃)₂C(CCH)? CH₃] = 70.7 ± 1.5, θH⁰_f((CH₃)₂?CCH,g) = 61.5 ± 2.0, and DH⁰[(CH₃)₂C(CCH)? H] = 81.0 ± 2.3, all in kcal/mol at 298°K. The stabilization energy of the dimethylpropargyl radical has been found to be 11.0±2.5 kcal/mol.     

Inductive effect of substituents on the dipole moment of α-substituted vinyl methyl ether

Dipole moments of several α-substituted vinyl methyl ethers R(OMe)C:CH₂; R = Me, Et, i-Pr, t-Bu, cyclopropyl, vinyl, Ph) have been determined by the Halverstadt-Kumler method in benzene solution at 293 K. The square of the total dipole moment μ_r was found to be a linear function of the Taft's inductive constant σ_r^*: μ_r²/D²=(0.619±0.033)+(1.092±0.10) σ_r^*. The inductive contribution of the substituent R on the total dipole moment may be expressed by the equation μ_j/D = ?0.52 σ^* + 0.25. This is in good agreement with the corresponding equation for the dipole moments of alkyl-substituted ethenes: μ_i/D = ?0.58 σ^* + 0.28 (based on dipole moments obtained by PCILO calculations).     

Simple isotopic product rules valid for absolute and integrated infrared intensities

Microwave spectra of CH¹⁸ OCOOH, CHOC¹⁸ OOH, CHOCO¹⁸ OH, ¹³ CHOCOOH and CHO¹³ COOH are reported and have been used in combination with data on CHOCOOH and CHOCOOD to determine the molecular structure as r(C=O)_ald. = 1.174 ± 0.006 Å, r(C=O)_acid = 1.203 ±0.006 Å, r(C—O) = 1.313 ± 0.010 Å, r(C—C) = 1.535 ± 0.005 Å, r(O—H) = 0.948 ± 0.004 Å, r(C—H) = 1.104 ±0.010 Å, ald. = 123.7 ± 0.4<, 相似文献   

Microwave spectra of isotopic glycolaldehydes,substitution structure,intramolecular hydrogen bond and dipole moment

The microwave spectra of ¹³CH₂OH-CHO, CH₂OH-¹³CHO, and CH₂OH-CH¹⁸O are reported and have been used in combination with previously published data on other monosubstituted glycolaldehydes to determine the substitution structure of the molecule as r(CO) = 1.209 Å, r(C-O) = 1.437 Å, r(C-C) = 1.499 Å, r(O-H) = 1.051 Å, r(C-H_ald) = 1.102 Å, r(C-H_alc) = 1.093 Å, r(O β H) = 2.007 Å, r(O β O) = 2.697 Å, ∠(C-CO) = 122°44', ∠(C-C-H_ald) = 115°16', ∠(C-C-O) = 111°28', ∠(C-O-H) = 101°34', ∠(C-C-H_alc) = 109°13', ∠(H-C-H) = 107°34', ∠(O-H β O) = 120°33', ∠(H β OC) = 83°41', and ∠(O-H, C0) = 24°14'. The intramolecular hydrogen bond and the other structural parameters are discussed and compared to related molecules. The dipole moment is redetermined to be μ_a = 0.262 ±0.002 D, μ_b = 2.33 ± 0.01 D, and μ_tot = 2.34 ± 0.01 D. Relative intensity measurements yielded 195 ± 30 cm^?1 for the C-C torsional fundamental and 260±40 cm^?1 for the lowest in-plane skeletal bending mode. Computations performed by the CNDO/2 method correctly predict the observed cis hydrogen-bonded conformer to be the energetically favoured one and in addition yield some indication of the existence of at least two other non-hydrogen-bonded forms of higher energy.     

Conformational Flexibility of the Methoxyphenyl Group Studied by Statistical Analysis of Crystal Structure Data

In a large sample of observed methoxyphenyl groups, the twist angle τ about the MeO-C_Ph bond measuring internal rotation of the MeO group shows a continuous distribution with maxima at (0°) (coplanar conformation) and (～90°) (perpendicular conformation). The preferred conformation of methoxyphenyl depends on the nature of the ortho--substituents: In general, it is coplanar in the case of one or two ortho-hydrogens, and perpendicular in the case of two substituents. The internal rotation of the MeO group is accompained by systematic variations in bond angles and bond distances: 1 if MeO is twisted out of plane, the bond angle CH₃? O? C_Ph decreases from 117.7°, until it reaches a minimum of 114.9° at τ = ±90°. The O? C? C angle which is syn to CH₃ for τ = 0° decreases from 124.6° to a minimum of 115.4° at τ = ±180°. These angles changes keep the nonbonded distance CH₃ …? ortho substituent maximal during internal rotation of MeO and tend to minimize the corresponding strain energy. (2) In the perpendicular conformation, the O-atom is ～ 0. 06 Å displaced from the Ph plane, O and CH₃ and being on opposite sides of this plane. In addition, small but systematic increases of bond lengths MeO? C_Ph and CH₃? O are observed. These variations indicate a decrease in conjugation with increasing twist angle. Their interdependence during twisting and the magnitudes of the changes are close values obtained by ab initio calculations.     

Microwave spectrum,conformational preference,intramolecular hydrogen bond,barrier to internal rotation,dipole moment,and centrifugal distortion of 1-fluoro-2-propanol

The microwave spectra of 1-fluoro-2-propanol, CH ₃CH(OH)CH ₂F, and one deuterated species, CH₃,CH(OD)CH₂F, have been investigated in the 18–30 GHz spectral region. Only one rotamer with an intramolecular hydrogen bond formed between the fluorine atom and the hydroxyl group was assigned. This conformation is also characterized by having the C-F bond approximately anti to the methyl group. The FCCO dihedral angle is 59 ± 2° and the HOCC dihedral angle is 58 ± 3°. Further conformations, if they exist, are at least 0.75 kcal mol^?1 less stable. Five vibrationally excited states belonging to four different normal modes were assigned and their fundamental frequencies determined. The barrier to internal rotation of the methyl group was found to be 2796 ± 50 cal mol^?1. The dipole moment is μ_a = 0.510 ± 0.009 D, μ_b = 1.496 t 0.026 D, μ_c = 0.298 ± 0.014 D, and μ_tot = 1.608 ± 0.030 D. Extensive centrifugal distortion analyses were carried out for the ground and the first excited state of the heavy-atom torsional mode and accurate values were determined for all quartic and two sextic coefficients.     

Bond dissociation energies and radical heats of formation in CH3Cl,CH2Cl2, CH3Br,CH2Br2, CH2FCl,and CHFCl2

An analysis of thermochemical and kinetic data on the bromination of the halomethanes CH_4–nX_n (X = F, Cl, Br; n = 1–3), the two chlorofluoromethanes, CH₂FCl and CHFCl₂, and CH₄, shows that the recently reported heats of formation of the radicals CH₂Cl, CHCl₂, CHBr₂, and CFCl₂, and the C? H bond dissociation energies in the matching halomethanes are not compatible with the activation energies for the corresponding reverse reactions. From the observed trends in CH₄ and the other halomethanes, the following revised ΔH°_f,298 (R) values have been derived: ΔH°_f(CH₂Cl) = 29.1 ± 1.0, ΔH°_f(CHCl₂) = 23.5 ± 1.2, ΔH_f(CH₂Br) = 40.4 ± 1.0, ΔH°_f(CHBr₂) = 45.0 ± 2.2, and ΔH°_f(CFCl₂) = ?21.3 ± 2.4 kcal mol^?1. The previously unavailable radical heat of formation, ΔH°_f(CHFCl) = ?14.5 ± 2.4 kcal mol^?1 has also been deduced. These values are used with the heats of formation of the parent compounds from the literature to evaluate C? H and C? X bond dissociation energies in CH₃Cl, CH₂Cl₂, CH₃Br, CH₂Br₂, CH₂FCl, and CHFCl₂.     

Structural evidence for Pπ-Pπ conjugation in aminosubstituted phosphaalkynes

The molecular structure of 2,2,6,6-tetramethylpiperidinophosphaalkyne was determined by the X-ray structural method. The main geometrical parameters are as follows: PC 1.559(2), N C(sp) 1.316(2) Å, PC N 178.9(1)°, with an almost planar trigonal bond configuration for the N atom and the chair conformation of the piperidine ring. Structural evidence for the nitrogen lone pair conjugation with the π-system of the triple bond was found to be different in phosphaalkynes PC-NR₂ and nitriles NC NR₂. Quantum-chemical ab initio calculations (HF/631G*) showed that this is caused by a different character of polarization of the PC and NC triple bonds.     

Microwave spectrum,conformation, intramolecular hydrogen bond,dipole moment, 14N quadrupole coupling constants and centrifugal distortion of 2flu

Microwave spectra of CH₂FCONH₂, CH₂FCOND(1)H(2), CH₂FCONH-(1)D(2), and CH₂FCOND₂ are reported. The stable form of the molecule is shown to possess a planar FCCONH₂ skeleton, with two out-of-plane hydrogens. The C-F and CO bonds are trans to one another and a weak intramolecular hydrogen bond is formed between the fluorine atom and the nearest amide group hydrogen atom stabilizing the identified rotamer. Other conformations are not present in concentrations exceeding 10% of the total. Nine vibrationally excited states were assigned. Six of these were attributed to the C-C torsional mode and one to the lowest in-plane bending mode. The first excited state of -NH_z out-of-plane deformation mode was tentatively assigned. Relative intensity measurements yielded 114±14 cm^?1 for C-C torsional mode and 239±20 cm^?1 for the in-plane bending mode. The dipole moment was determined asμ_a = 1.27±0.01 D, μ_b = 1.67±0.02 D, and μ_tot = 2.10±0.02 D, while the ¹⁴N quadrupole coupling constants were found to be χ_aa = 1.6±0.2 MHz, χ_bb = 1.6±0.2 MHz and χ_cc = ?3.2±0.3 MHz.     

Studies on metal—acetylene complexes : VI. Crystal and molecular structure of [diethylbis(1-pyrazolyl)borato]methyl(1-phenylpropyne)-platinum(II), [(C2H5)2B(N2C3H3)2](CH3)Pt(C6H5CCCH3)

The crystal and molecular structure of [diethylbis(1-pyrazolyl)borato]methyl(1-phenylpropyne)platinum(II), ([(C₂H₅)₂B(N₂C₃H₃)₂](CH₃)Pt(C₆H₅CCCH₃)), has been determined by a single crystal X-ray diffraction study using diffractometer techniques. The compound crystallizes in the monoclinic space group P2₁/c with a = 13.239(6), b = 11.077(5), c = 15.619)7) Å and β = 114.53(2)°. The observed density of 1.71(2) g cm^?3 agrees well with the calculated value, 1.687 g cm^?3, assuming four molecules in the cell. A conventional agreement factor of 0.036 was obtained by least-squares refinement on F using 3289 observations and 194 variables. The coordination about the platinum atom is square planar, if the acetylene is assumed to occupy one coordination site. The substituents of the acetylene are cis-bent away from the Pt atom, the methyl substituent by 17.7(1.0)°, and the phenyl substituent by 21.2(9)°. The coordinated triple bond length is 1.227(10) Å. These results indicate that the acetylene is moderately perturbed on coordination, consistent with the observation that Δν(CC) is 211 cm^?1. The conformation of the ring formed by the bidentate polypyrazolylborate ligand is that of a “shallow” boat. One of the methylene H atoms on the ethyl substituents on the polypyrazolylborate ligand when placed in an idealized position is 2.65 Å distant from the Pt atom.     

Synthese und Kristallstruktur des Nitridokomplexes [Na-15-Krone-5]2[MoNF4]2 · 2 CH3CN

Synthesis and Crystal Structure of the Nitrido Complex [Na-15-crown-5]₂[MoNF₄]₂ · 2 CH₃CN The title compound is synthesized by the reaction of [MoCl₄(NSCl)]₂ with excess NaF in boiling acetonitrile in the presence of the crown ether 15-crown-5. [Na-15-crown-5]₂[MoNF₄]₂ · 2 CH₃CN forms yellow crystals, which were characterized by an X-ray structure determination. Space group P1 , Z = 1. Lattice dimensions at ?90°C: a = 855.5, b = 1 069.9, C = 1 143.5 pm, α = 105.71°, β = 95.29°, γ = 102.25° (4 096 independent observed reflexions, R = 0.039). Short Na…?F contacts of 234 pm with the four axial fluoro ligands of the dimeric anion [MoNF₄]₂^2? allow formulation of a triple ion. The centrosymmetric anion is dimerized by bent fluoro bridges with Mo? F distances of 198 and 245 pm. The long Mo? F distances of the MoF₂Mo ring are in transposition to the nitrido ligands, the bond lengths of which (165 pm) correspond to triple bonds.     

NMR-untersuchungen an einigen 1,3-diinen

¹³C, ²⁹Si and ¹¹⁹Sn NMR data (chemical shifts and coupling constants) are reported for 1,3-diynes RCCCCR′ (R = R′ = H, t-C₄H₉, Si(CH₃)₃, Sn(CH₃)₃; R = Si(CH₃)₃, R′ = Sn(CH₃)₃). The data are in agreement with an increased polarity of the SnC bond in the 1,3-diynes as compared with alkynylstannanes.     

Molecular determinants for drug—receptor interactions: Part 2. An ab initio molecular orbital and dipole moment study of the novel nootropic agent piracetam (2-oxopyrrolidin-1-ylacetamide)

From the ab initio molecular energies of the possible conformers and from a classical dipole moment analysis of 2-oxopyrrolidin-1-ylacetamide (υ = 4.02 D in dioxan at 30.0°C), the preferred conformation in solution of this novel nootropic agent has been determined. The exocyclic NCH₂ bond is rotated in one sense by 90° and the exocyclic CH₂C bond rotated in the same sense by 120° from the “planar” (OO)-cis conformation. The structures of the two enantiomers in solution differ from that of the crystalline molecule.     

Microwave spectrum,conformation, barrier to internal rotation,centrifugal distortion,and dipole moment of methylpropargyl ether

The microwave spectrum of methylpropargyl ether, CH₃OCH₂CCH, has been investigated in the 11.9–26.5 GHz region. Only the gauche rotamer with a dihedral angle of 68° ± 2° from the syn position was assigned. Other forms are not present in concentrations exceeding 10 % of the total. The barrier to internal rotation of the methyl group was determined to be 2512 ± 75 cal mol^?1. The dipole moment components are μ_a = 0.290 ± 0.003 D, μ_b = 0.505 ± 0.012 D, and μ_c = 1.016 ± 0.003 D. The total dipole moment is 1.171 ± 0.013 D. Extensive centrifugal distortion analyses have been carried out for the ground as well as for two vibrationally excited states. For the ground state, transitions up to J = 77 were assigned and a large centrifugal distortion exceeding 9 GHz enabled the determination of accurate quartic and significant sextic distortion coefficients.     

Allycyanide gauche revisited

The microwave spectrum of the ground state of the gauche rotamer of allylcyanide (CH₂=CHCH₂ CN) has been remeasured. The obtained rotational constants A = 19 707.9 ± 0.1, B = 2 619.74 ± 0.05 and C = 2 497.43 ± 0.05 (in MHz) were in good agreement with a structural model. The dipole moment components were also fitted as |μ_a| = 3.50 ± 0.05, |μ_b| = 1.70 ± 0.02 and |μ_c| = 0.19 ± 0.04 (in Debye). The results are in both cases in good agreement with a CCCC dihedral angle near the expected 120°.     

Polysulfonylamine. VII. Aliphatische Trisulfonylamine

Polysulfonyl Amines. VII. Aliphatic Trisulfonyl Amines The compounds N(SO₂R¹)₂(SO₂R²) with R¹ = R² = CH₃ ( 2a ), R¹ = R² = C₂H₅ ( 2b ) and R¹ = CH₃, R² = C₂H₅ ( 2c ) are prepared by cleavage of aminostannanes (CH₃)₃SnN(SO₂R¹)₂ with sulfonyl chlorides R²SO₂Cl. A simple synthesis of 2a from AgN(SO₂CH₃)₂ and CH₃SO₂Cl is described. From the vibrational spectra of 2a , evidence is obtained for a planar NS₃ group in this compound. X-ray structure determinations of 2b and HN(SO₂C₂H₅)₂ ( 3 ) are reported. In 2b , the NS₃ group is approximately planar (S? N? S bond angles 119.0 ± 0.6°, sum of bond angles at N 356.9°); the S? N bond lengths of ca. 173 pm indicate a bond order of 1. In compound 3 , the nitrogen atom has a planar coordination (S? N? S angle 125.3°, sum of bond angles at N 359.3°), the S? N bond lengths of ca. 165 pm correlate with a bond order of 1.3? 1.4.     

The kinetics and equilibrium of the gas-phase reaction CH3CF2Br + I2 = CH3CF2I + IBr the C-Br bond dissociation energy in 1,1-difluorobromoethane

The kinetics and equilibrium of the gas-phase reaction of CH₃CF₂Br with I₂ were studied spectrophotometrically from 581 to 662°K and determined to be consistent with the following mechanism: A least squares analysis of the kinetic data taken in the initial stages of reaction resulted in log k₁ (M^?1 · sec^?1) = (11.0 ± 0.3) - (27.7 ± 0.8)/θ where θ = 2.303 RT kcal/mol. The error represents one standard deviation. The equilibrium data were subjected to a “third-law” analysis using entropies and heat capacities estimated from group additivity to derive ΔH_r° (623°K) = 10.3 ± 0.2 kcal/mol and ΔH_rr (298°K) = 10.2 ± 0.2 kcal/mol. The enthalpy change at 298°K was combined with relevant bond dissociation energies to yield DH°(CH₃CF₂ - Br) = 68.6 ± 1 kcal/mol which is in excellent agreement with the kinetic data assuming that E₂ = 0 ± 1 kcal/mol, namely; DH°(CH₃CF₂ - Br) = 68.6 ± 1.3 kcal/mol. These data also lead to ΔH_f°(CH₃CF₂Br, g, 298°K) = -119.7 ± 1.5 kcal/mol.     

Effect of the Electronic Structure of Substituents at the Silicon Atom on the Structure and Bond Energies of Methylhydrosilanes and Silenes

Ab initiocalculations with full geometry optimization were performed for methylhydrosilanes R₂HSiCH₃, dimethylsilanes C₂Si(CH₃)₂, and silenes R₂Si = CH₂ (R = H, CH₃, SiH₃, CH₃O, NH₂, Cl, F). The enthalpies of dehydrogenation methylhydrosilanes into silenes and of dehydrocondesation of methylhydrosilanes into dimethylsilanes were calculated. The enthalpies of dehydrogenation and dehydrocondensation increase with the electronegativity of substituent R. The Si-C and Si = C bond energies were calculated. As the electronegativity of the substituent increases, the Si-C bond shortens and strengthens, while the Si = C bond shortens and weakens.