摘要:

FT-IR transmission spectra of liquids in well-made and firmly held cells with KBr or NaCl windows are usually very reproducible except that their baselines often show unexpected variations. To obtain absolute absorption intensities from these spectra such baseline differences must be corrected. The problem is illustrated with absorbance spectra of the 1036 cm−1band of benzene and the absorption index spectra calculated from them via the National Research Council of Canada program 46. Distinction is made between the experimental absorbance spectrum, the ideal experimental absorbance spectrum, and the absorbance spectrum, and a soundly-based method to correct the baselines is presented. We describe a modification of the NRC program 46 that effects the correction and calculates, on a laboratory computer, the complex refractive indices from a transmission spectrum of a liquid.The method is applied to 15 transmission spectra of the 1036 cm−1band of benzene. It improves the agreement between the 15 peak absorption index values obtained from these spectra from 4% to 2.5%, and improves the agreement between the baseline absorption index values from 5% to 0.01%, all percentages being of the peak value. A table of the average real and imaginary refractive indices and molar absorption coefficients is given. The average peak absorption index value and the area under the band agree closely with those obtained in 1980, and with earlier values from transmission or ATR measurements. These areas are all distinctly lower than those calculated from early measurements of dispersion. These results form the basis for a possible recommendation to the International Union of Pure and Applied Chemistry for a secondary intensity standard. We encourage others to measure this band to help ascertain that systematic errors do not significantly influence our results.To summarise our numerical results at 25 ± 1 °C and 1.0 cm−1nominal resolution: The average peak absorption index is 0.0710 with a 95% confidence limit of 0.0004, which is within 0.0001 of the less precise value measured in 1980 with a dispersive instrument that was calibrated against a primary intensity standard. The area under the absorption index band between 1095.8 and 912.6 cm−1is 1.358 cm−1with a 95% confidence limit of 0.009 cm−1, and that under the molar conductivity band is 3.69 ± 0.03 km/mol. These areas convert to 15.64 ± 0.1 km/mol and 6.79 ± 0.05 km/mol for the more commonly used measures of integrated absorption, the areas under the Naperian and decadic molar absorption coefficients, respectively.Keywords: infrared, absorption intensity, optical constants, FT-IR spectroscopy, benzene (liquid), intensity standard.

ISSN:0008-4042    

DOI:10.1139/v91-236

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Vibrational circular dichroism (VCD) spectra of an exceptionally simple chiral three-membered ring molecule, (S,S)-[2,3-2H2]oxirane, have been obtained for samples in the gas phase and in C2Cl4and CS2solution. The VCD band contours for the gas phase samples of this molecule withC2symmetry follow those of the absorption spectra, with the modes ofAsymmetry species giving rise to bands with two sharp Q-branch maxima on either side of a distinct minimum, and the modes ofBsymmetry species giving rise to bands with a strong central Q-branch. The signs and relative intensities of the VCD bands can be understood in terms of chiral perturbation by the deuterium substitution of the vibrational modes of the parent oxirane molecule, which mixes modes ofA1andA2symmetry, and modes ofB1andB2symmetry, thus introducing angular charge oscillation about the direction of linear charge oscillation.Keywords: vibrational circular dichroism, oxirane, chiral perturbation.

ISSN:0008-4042    

DOI:10.1139/v91-237

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The energies and geometries of the low-lying electronic states of formylmethylene have been calculated at the SCF and CI levels using 6-31G** basis set. In agreement with previous reports and accumulated experimental observations, the ground state is the carbenoid triplet with a planar geometry. Also, in agreement with all previous single configuration SCF and CI calculations, the lowest excited singlet state is computed to be the nonplanar closed shell carbenoid structure. In contrast, accumulated experimental evidence along with previously reported MC–SCF results require this state to be planar. The present calculation predicts the existence of a (σ–σ) 1,3-diradical1A′ state, which appears to be identical to the MC–SCF lowest singlet state, but this state lies some 11 kcal/mol above the closed shell carbenoid1A state. Apparently, single configuration SCF methods are inadequate for the correct description of the electronic manifold of formylmethylene.Keywords: formylmethylene, singlet and triplet states,abinitioMO calculations, conformers, molecular structure.

ISSN:0008-4042    

DOI:10.1139/v91-238

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The peptide Asp-Glu-Lys-Ser has been studied using the software package ALTA:maPS, with simulation of the pH conditions and the influence of the solvent, in order to investigate the possible stabilization of its structure by a hydrogen bond and a salt-bridge. The theoretical results confirm the existence of a salt-bridge, between the side chains of the Glu and Lys residues, in aqueous solution at neutral pH but not at acid pH, in agreement with the observation from NMR studies. On the other hand the calculations indicate that no hydrogen bond exists in aqueous solution at either pH. The fact that an in-vacuum simulation shows the existence of a hydrogen bond between the peptidic CO group of the Asp residue and the peptidic NH group of the Ser residue suggests that a molecular dynamics study might confirm that a small percentage (less than 25%) of the peptide molecules may exist as an H-bonded conformer, as postulated from the NMR results.Keywords: peptides, conformation, theoretical prediction.

ISSN:0008-4042    

DOI:10.1139/v91-239

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The frequency of the so-called "amide I" band (amide C=O stretching vibration,vC=O) of proteins is discussed in terms of the dihedral angles of the various secondary structures present within proteins. We propose that in the case of intra- or intermolecular hydrogen-bonded amide carbonyl groups the frequency of this absorption can be directly related to the, ψ angles of the amide moieties for the major secondary structures. Amide I bands at frequencies above those found for non-hydrogen bonded amide carbonyl groups are rationalized in terms of a change in the third dihedral angle, ω. Rotation around the amide C—N bond in sterically demanding structures, such as turns where ω deviates from 180°, is expected to cause an increase in the electron density of the amide carbonyl groups and so increasevC=O to frequencies greater than that seen for unperturbed carbonyl groups.Keywords: FT-IR spectroscopy, proteins, structure, dihedral angles.

ISSN:0008-4042    

DOI:10.1139/v91-240

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The photoreduction of xanthone with diethylaniline in cyclodextrin cavities was studied at 77 K and room temperature by a time-resolved ESR technique. The radical pair observed in β- and γ-cyclodextrins showed inverted spin polarization compared to that of precursor excited triplet xanthone. This result is rationalized by taking account of the fixed orientation of the radical ion pair in the cyclodextrins. Frozen aqueous solutions and dried powder-like samples provided similar results. The spectrum of the radical pair was also detected in an aqueous solution of β-cyclodextrin at room temperature.Keywords: cyclodextrins, xanthone, spin polarization, radical ion-pair, time-resolved ESR.

ISSN:0008-4042    

DOI:10.1139/v91-241

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Fourier transform infrared spectroscopy has been used to investigate the effect of added cations and anions on CO adsorption on platinum and palladium hydrosols. In general, anion effects on CO adsorption may be classified into three categories. In the first category, poisoning anions (e.g. CN−, SH−) block the surface against CO adsorption so that infrared spectra exhibit either no ν(CO)adsbands or ν(CO)adsbands at lower frequencies (< 2070 cm−1). In the second category, inert anions (e.g. the halides, citrate, EDTA2−) have no apparent effect on ν(CO)ads. The third category is characterised by anions such as PO43−, CO32−, or stearate ion which affect ν(CO)adsvia the pH change that the dissolved anions cause in the metal hydrosol dispersion medium. Infrared spectra of platinum and palladium hydrosols in cyanide media were found to exhibit bands due to cyanate, tetracyanoplatinate(II), and tetracyanopalladate(II) arising from cyanide corrosion of the metal hydrosols. Ultraviolet/visible spectra of iodide ion in platinum hydrosols indicated that iodoplatinate(II) species had been produced. The band shape of ν(CO)adsat 2070 cm−1in platinum hydrosols is found to be sensitive to the state of dispersion of the hydrosol particles. Bands due to CO adsorbed on hydrosol particles in either a completely or partially aggregated state are weaker and broader with a decreased value of ν(CO)adsrelative to that of the unaggregated hydrosol.Keywords: platinum hydrosols, palladium hydrosols, CO adsorption, infrared spectroscopy, added salts.

ISSN:0008-4042    

DOI:10.1139/v91-242

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Thermal lensing and photoacoustic spectra of ν3, ν1 + 3, and ν1 + ν2 + ν3vibrational transitions of gaseous acetylene were observed by a tunable pulsed IR laser converted from a pulsed dye laser. Application of a pulsed IR laser to photothermal spectroscopy is demonstrated.Keywords: thermal lensing spectra, photoacoustic spectra, pulsed tunable infrared laser.

ISSN:0008-4042    

DOI:10.1139/v91-243

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The high resolution infrared spectrum of LiF has been measured in emission with the McMath Fourier transform interferometer at Kitt Peak. A total of 800 lines with ν = 1 → 0 to ν = 8 → 7 of the main isotopomer,7LiF, and 250 lines with ν = 1 → 0 to ν = 3 → 2 of the minor isotopomer,6LiF, were observed. These ro-vibrational transitions and the pure rotational transitions from the literature were fit to a set of Dunham coefficientsYijand a set of mass-reduced Dunham coefficientsUij. The same spectrum shows 13 pure rotational emission transitions of HF in the vibrational ground state withJ = 13 → 12 toJ = 25 → 24. These transitions were used to determine an improved set of rotational constants for HF.Keywords: infrared spectra, LiF, HF.

ISSN:0008-4042    

DOI:10.1139/v91-244

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

A series of computer routines for performing normal coordinate calculations of harmonic vibrational frequencies is described. These are designed for operation on current-generation computers having direct-access storage. Example results are given for the chain isomer of disulfur difluoride.Keywords: vibrational force field calculations, normal coordinate calculations, harmonic force fields.

ISSN:0008-4042    

DOI:10.1139/v91-245

出版商:NRC Research Press    

年代:1991

数据来源: NRC