Evidence of the weakness of the OH⋯F hydrogen bond from a conformational study of 3-fluoro-1-propanol by microwave spectroscopy

The rotational spectra of the OH and OD isotopic species have been observed for three rotamers of 3-fluoro-1-propanol. One of them (HBC form) displays an internal hydrogen bond with a distorted chair conformation of the six-membered ring. The other two rotamers have the oxygen atom gauche with respect to the C₂C₃ bond, the hydroxyl hydrogen trans with respect to the C₁C₂ bond and the fluorine atom gauche (GGT form) and trans (TGT form), respectively, with respect to the C₂C₁ bond. The energies of the vibrational ground states of the HBC and TGT forms are ～0.4 and 1.0 kcal/mole higher than that of the GGT form, respectively (from relative intensity measurements). The hydrogen bond is therefore rather weak in this compound. With compounds capable of forming OH?O or OH?N bonds, the conformation appropriate for hydrogen bonding is normally the most stable form. Several excited states have been analyzed for the TGT and GGT rotamers in order to have additional data with respect to the potential function for the internal rotation about the C₃C₂ bond.     

Infrared spectrum of propargylamine

Infrared absorption spectra of propargylamine in the gas phase and in the low-temperature Ar matrix were observed. The vibrational analysis was made with the help of an ab initio MO calculation. A set of effective force constants was derived which reproduces the observed frequencies. The spectrum indicated the occurrence of one rotational isomer, namely trans around the CN bond.     

Internal hydrogen bond,torsional motion,and molecular properties of 2-methoxyethylamine by microwave spectroscopy: Methyl barrier to internal rotation for 2-methoxyethanol

The microwave spectra of four substituted isotopic species of 2-methoxyethylamine (NH₂, NHD, NDH, ND₂) have been assigned. The molecule is found to exist in a gauche form with an intramolecular hydrogen bond of the NH?O type. The four possible sets of the amino hydrogen r_s corrdinates give different H?H distances, probably because the -NH₂ group is involved in large amplitude vibrations and because of changes in the heavy atom positions arising from the deuteration of the hydrogen bond. For the most abundant species many vibrational states have been analyzed and assigned to the two possible CO torsions in the molecule. A value V₃ = 3150 ± 50 cal/mol was found for the methyl torsional barrier and V₁ = 9 ± 3 kcal/mol for the other CO torsional barrier. A third set of observed vibrational satellites is probably assignable to the CC torsion. The determination of the dipole moment and of the quadrupole coupling constants gave values which were not in good agreement with those predicted from nonhydrogen bonded molecules. In addition a value V₃ = 3100 ± 100 cal/mol was calculated for the CH₃ torsional barrier in the related 2-methoxyethanol, using previous experimental data (Canad. J. Chem.50, 1149–1156 (1972)).     

Control of oxidation selectivity by alkyl substitution: the reactions of t-butyl iodide and t-butanethiol on Rh(111)-p(2 × 1)O

We report the formation of adsorbed t-butoxy by addition of transient t-butyl radicals to surface oxygen, in the reactions of t-butyl iodide and t-butanethiol on Rh(111)-p(2 × 1)O. t-Butanethiol reacts by cleavage of the weak SH bond, forming adsorbed t-butyl thiolate upon adsorption at 100 K. t-Butyl thiolate subsequently forms t-butanol, isobutene and water in the range of 325–345 K via a transient t-butyl radical formed by cleavage of the CS bond. Similarly, t-butyl iodide reacts by cleavage of the CI bond to yield isobutene and t-butanol at 260 K. In both cases, isobutene is proposed to form via a combination of rapid β-hydrogen elimination of the t-butyl radical and decomposition of t-butoxide formed from addition of the radical to O_a. t-Butanol is produced via a competing hydrogenation of t-butoxide. The relative reaction temperatures correlate with the CI and CS bond strengths, indicating that homolytic CI and CS bond scission limit the rates of the reaction. Some t-butoxy remains on the surface up to 380 K, and is identified using high resolution electron energy-loss and temperature-programmed reaction spectroscopies. These studies demonstrate that the product distributions for reactions of alkyls on oxygen-covered Rh(111) can be manipulated by altering the molecular structure. Specifically, we were able to induce partial oxidation of the alkyl radical to the corresponding alcohol by effectively eliminating the pathway for dehydrogenation at the carbon adjacent to the oxygen via alkyl substitution.     

Structure determination of ethylene adsorbed on Ag(100) surface by multiple-scattering cluster method

The carbon K-shell near edge X-ray absorption fine structure (NEXAFS) spectra of gas-phase ethylene molecules and ethylene adsorbed on Ag(100) have been calculated by the multiple scattering cluster method (MSC) for the first time. The calculated results show that carbon-hydrogen scattering resonances exist in the carbon K-shell NEXAFS spectra for both gas-phase ethylene molecules and ethylene adsorbed on the metal surface. By a comparison between the theoretical results and experimental spectra, we have found that ethylene molecules are adsorbed on the Ag(100) surface taking a four-fold hollow-site with the direction of the CC bond parallel to the [001] or [010] direction, the bond lengths of CC and AgC are equal to 1.40 ± 0.02 and 1.96 ± 0.03 Å, respectively. These results can be compared with that of the C₂H₄-Cu(100) system.     

NMR study of the Rα structures of 2-chloro- and 2-bromothiophene partially oriented by nematic liquid crystals

The ¹H NMR spectra with the ¹³C satellites and the '3C spectra of 2-chloro- and 2-bromothiophene in nematic Merck ZLI 1167 and ZLI 1132 liquid crystal solutions were recorded and analyzed. The 'H spectra and three of the four ¹³C-satellite spectra appeared to be deceptively simple and prevented the determinations of certain individual coupling constants. As a consequence, the problems became under-determined. However, the use of two liquid crystal solvents and the assumption of unchanged structures in the solutions made it possible to determine the complete r_αr_α structures, although the linear dependence of some dipolar coupling constants led to more poorly determined groups of equations than was expected. The CH and CC distances, bond angles, and orientation matrix elements are reported for both 2-chloro- and 2-bromothiophene.     

Fast leed intensity measurements from Ni(100)c(2 × 2)CO

The Ni(100)c(2 × 2)CO surface structure has been investigated by very fast LEED intensity measurements using a computer controlled television method. It turns out that the intensity spectra are strongly influenced by intolerably long measuring times during which the primary electron beam impinges onto the surface. The spectra have been taken within 16 sec at 100 K immediately after termination of the adsorption process for all beams simultaneously. They are compared with other measurements and with Pendrys model calculations for a CO molecule bonded linearly on top of a Ni atom with straight molecular axis normal to the surface. Using the r-factor formalism for theory-experiment comparison the bond length results to be 1.15 ± 0.1 Å for CO and 1.80 ± 0.1 Å for NiC. This is in agreement with the results of other methods and removes some discrepancies with those of earlier LEED experiments.     

Oxide-supported metals with molecular structures: Evidence of the metal-oxygen bond from Raman and inelastic electron tunneling spectra of [Re(CO){in3}{OMG}{HOMg}2] and [Re(CO)3{OAl}{HOAl}2]

Raman and inelastic electron tunneling spectra of the anchored rhenium complexes [Re(CO){in3}{OMG}{HOMg}₂] and [Re(CO)₃{OAl}{HOAl}₂] are reported, providing the first vibrational spectroscopic evidence of the metal-oxygen bond in mononuclear oxide-bound organometallic complexes.     

Microwave substitution structure of the amine group in Ortho-chloroaniline,C6H4CINH2

The microwave spectra of amine deuterated species of [2-³⁵Cl] and [2-³⁷-Cl]aniline, C₆H₄ClNH₂, have been observed. The rotators associated with the first two states of the amino group inversion have been assigned. A geometric structure has been calculated from the moments of inertia of all isotopic species in the ground state, indicating a small out-of-plane angle (34.5°), and a torsion around the CN bond of the NH₂ group. This deformation is explained by the formation of a hydrogen bond NH … Cl.     

FTIR spectra of the HF complexes with alkyl cyanides in solid argon

Ethyl-, i-propyl, and t-butyl cyanide were separately condensed at 9–12K with HF in excess argon producing complexes of the form RCNHF. Diffusion of HF was accomplished by warming the matrix above 18K which produced 1:2 and 1:3 complexes RCN(HF)₂ and RCN(HF)₃. The HF submolecule stretching frequencies for the 1:1 complexes were very similar, giving absorptions at 3515, 3530 and 3520 cm^?1, respectively, while the HF librational modes were observed centered around 650 cm^?1. The CN symmetric stretch in the complexes was perturbed to higher energy in each case implying a strengthening of the CN bond, with the C-C₃ symmetric stretch in t-butyl cyanide also shifted to higher energy.     

INDO-MO study of γ-CH formyl-proton couplings in aliphatic α-formyloxy radicals

In aqueous solution at approximately 25°C, simple aliphatic formate radicals of general formula ·C(X₁)(X₂)OCHO, where an (X-) group is an H- or a substituent such as CH₃-, generally display γ-CH formyl-proton couplings. The size of these couplings depends on the identity of the (X-) groups. An earlier, detailed INDO-MO-SCF investigation of the radical ·CH₂OCHO carried out by considering all conceivable bond rotations suggested a simple interpretation of this dependence, namely, that it followed from the likelihood that substituents at C_α would influence the conformational preferences and these would determine the formyl-proton coupling according to the W-plan rule. As a continuation of this earlier work, the present paper reports the results of a comprehensive INDO-MO-SCF study of the α-formyloxy radicals ·CH(CH₃)OCHO, ·CH(CH₂CH₃)OCHO, ·CH(CH(CH₃)₂)OCHO, and ·C(CH₃)₂OCHO. We found the only bond rotations which appreciably affected the γ-CH formyl-proton coupling in these species to be those about C_αO_β and O_βC_γ. The other results of the present calculations confirm the tentative conclusions of the earlier study about the origin of the dependence of the formyl-proton couplings on the nature of the substituents at C_α. Furthermore, by examining the INDO data in comparison with the experimentally observed formyl-proton couplings, we have been able to assign preferred radical conformations for all members of the ·C(X₁)(X₂)OCHO series, in general agreement with the known conformational preferences of the parent formate molecules, HC(X₁)(X₂)OCHO.     

Microwave spectrum,structure, and nuclear quadrupole coupling of cis-1-chlorobutadiene-1,3

The microwave spectra of the two natural isotopic species of cis-1-chlorobutadiene-1,3, CH³⁵ClCHCHCH₂ and CH³⁷ClCHCHCH₂, together with all monosubstituted species with deuterium or ¹³C isotopes have been measured and assigned and the complete substitution structure has been determined. The spectral region investigated was between 18 and 40 GHz. The molecule was found to be planar and the following values were found for the principal parameters: r(CC)_chlorovinyl = 1.327(6), $\text{r(}\text{CC}\text{)}{}_{\mathrm{<mtext>vinyl</mtext>}}\text{= 1.343(2)}\text{A}\text{?}$, <(CCC)_chlorovinyl = 126.5(3)° and <(CCC)_vinyl = 123.0(7)°. The nuclear quadrupole coupling constants, χ_aa and χ_bb, for the parent molecule were calculated and further used to estimate the symmetry of the field gradient around the CCl bond. Force-field calculations were used to predict the centrifugal distortion constants and inertial defect of some isotopic species. Thermodynamic functions were calculated for cis- and trans-1-chlorobutadiene-1,3 and used to estimate the energy difference between them.     

Microwave spectrum,dipole moment,and structure of 3-aminopropanol

The microwave spectra of 3-aminopropanol and three of its deuterium substituted isotopic species have been investigated in the 26.5 to 40 GHz frequency region. The rotational spectrum of only one conformer has been assigned in which presumably a hydrogen bond of the OH---N type exists. The rotational spectra of a number of excited vibrational states have been observed and assignments made for some of these excited states. The average intensity ratio for the rotational transitions between the ground and excited vibrational states indicates that the first excited state is about 120 cm^?1 above the ground state.and the next higher state is roughly 200 cm^?1 above the ground vibrational state. The dipole moment was determined from the Stark effect measurements to be 3.13 ± 0.04 D with its principal axes components as |μ_a| = 2.88 ± 0.03 D, |μ_b| = 1.23 ± 0.04 D and |μ_c| = 0.06 ± 0.01 D. The possibility of another conformer where the hydrogen bond could be of NH---O type was explored, but the spectra of such a conformer could not be identified.     

Blue photoluminescence from SiGeSiO2 co-sputtered films

A blue photoluminescence band centered at 440 nm was observed from SiGeSiO₂ co-sputtered films at room temperature. This band gains intensity after the film was annealed at a temperature around 900°C in N₂ atmosphere. From analysis of photoluminescence excitation, Raman and X-ray photoelectron spectra, it turns out that the luminescence is probably from some interfacial state between Si_1−xGe_x nanoparticles and the SiO₂ matrix.     

Study of adsorption phenomena on aluminium by interatomic Auger transition spectroscopy: II. CO adsorption onto Al

Adsorption of carbon monoxide on aluminium is studied by the combination of in situ evaporation of Al and the Auger electron spectroscopy. New Auger signals which are attributed to interatomic Auger transitions in Al₄C₃ type AlC bonds are found. It is revealed that two adsorption stages exist in the adsorption process, i.e., that after the formation of the AlC bond takes place, the adsorbed layer consisting of AlC and AlO bonds is formed.     

The microwave spectrum and structure of fulvene

The microwave spectra of four ¹³C and two monodeutero species of fulvene have been recorded and analysed. Earlier data for the parent species has also been slightly refined. The final structure which emerges for fulvene is essentially three weakly coupled ethylenic entities with bond lengths: C₁C₂ = 1.470Å C₂C₃ = 1.355Å, C₃C₄ = 1.476Å and C₁C₆ = 1.3485Å. The CH bond distances and ring angles are close to expected values.     

Molecular structure of cyclopropenylidene,HCCCH from the millimeter wave spectra of its isotopomers

The rotational spectra of the two ¹³C and of the D monoisotopic substitutions of cyclopropenylidene C₃H₂ have been measured up to 300 GHz. The determination of the rotational and centrifugal distortion constants of four isotopomers has led to a complete substitution structure. The values of the two CC bond lengths which define the three-membered ring have been improved by using Watson's “mass-dependence” (r_m) method.     

The microwave spectrum of lactic acid

The microwave spectrum of lactic acid has been identified with aid of double resonance techniques. Satellite spectra have been tentatively assigned to the two CC torsional modes. From deuterium substitution into the two hydroxyl groups the conformation was determined: the molecule shows a hydrogen bond from the α-hydroxyl group to the carbonyl oxygen atom. The c coordinate of the hydrogen atom involved has the appalling imaginary value 0.31i, thus once more casting doubt on the reliability of the substitution method for accurate structure determination.     

Pyrolysis of amines: Infrared spectrum of C-cyanomethanimine

The gas-phase infrared absorption spectrum of C-cyanomethanimine, NHCHCN, was observed. This molecule was produced from the thermal decomposition of dimethylcyanamide. The trans form was the more abundant of the two rotational isomers, cis and trans, which define the positions of the two hydrogen atoms around the CN double bond. An ab initio force field was calculated and was used to assign the vibrational modes.     

Formic acid anhydride: Matrix infrared spectra of five isotopic species,vibrational analysis,empirical and ab initio harmonic force field and thermodynamic functions

Argon matrix infrared spectra of formic acid anhydride (CHOOCHO, CDOOCHO, CDOOCDO, and CH¹⁸O¹⁸OCH¹⁸O) are presented. Assignments and normal coordinate analysis yield valence force constants (VFF I) reflecting a significant dependence on the conformation of the formyl groups. A similar dependence is borne out by force constants obtained from an ab initio SCF treatment. The quantum chemical force constants qualitatively confirm Badger's rule. It is shown that a second valence force field (VFF II) may be found which is in qualitative agreement with the quantum chemical force constants and also reproduces the observed frequencies just as well as VFF I. The stretching force constants appear to be enhanced for bonds which are trans to another bond.