Infrared Band Intensities Research Articles

Vibrational and Electronic Absorption Spectroscopy of Dibenzo[b,def]chrysene and Its Ions

The vibrational and electronic absorption spectra of dibenzo[b,def]chrysene (DBC) and its ions in argon matrixes have been recorded. Assignment of the observed infrared (IR) bands has been made by comparison with the density functional theory (DFT) computations of harmonic vibrational frequencies (with 6-31G(d,p) or 6-311+G(d,p) basis sets). Extensive time-dependent (TD) DFT calculations of vertical excitation energies have aided in the assignment of the experimental electronic absorption transitions. In general, the theoretical predictions are in good agreement with the observed ultraviolet and visible bands. By correlating IR and UV-visible band intensities (after UV photolysis), it has been shown that both DBC cations and anions are formed. The IR band intensity distributions of the DBC ions differ markedly from neutral DBC. A synthetic spectrum composed of neutral, cationic, and anionic DBC contributions compares reasonably well with the interstellar features of the "unidentified infrared" (UIR) bands from the reflection nebula NGC 7023. Finally, it is shown that the electronic absorption bands of the DBC ions lie in close proximity to several of the diffuse interstellar visible absorption bands (DIBs).

Vibrational Spectroscopic Properties of Hydrogen Bonded Acetonitrile Studied by DFT

Vibrational properties (band position, Infrared and Raman intensities) of the acetonitrile C[triple bond]N stretching mode were studied in 27 gas-phase medium intensity (length range: = 1.71-2.05 angstroms; -deltaE range = 13-48 kJ/mol) hydrogen-bonded 1:1 complexes of CH3CN with organic and inorganic acids using density functional theory (DFT) calculations [B3LYP-6-31++G(2d,2p)]. Furthermore, general characteristics of the hydrogen bonds and vibrational changes in the OH stretching band of the acids were also considered. Experimentally observed blue-shifts of the C[triple bond]N stretching band promoted by the hydrogen bonding, which shortens the triple bond length, are very well reproduced and quantitatively depend on the hydrogen bond length. Both predicted enhancement of the infrared and Raman nu(C[triple bond]N) band intensities are in good agreement with the experimental results. Infrared band intensity increase is a direct function of the hydrogen bond energy. However, the predicted increase in the Raman band intensity increase is a more complex function, depending simultaneously on the characteristics of both the hydrogen bond (C[triple bond]N bond length) and the H-donating acid polarizability. Accounting for these two parameters, the calculated nu(C[triple bond]N) Raman intensities of the complexes are explained with a mean error of +/- 2.4%.

UV-induced single and double hydrogen-atom migrations in 3,6-diimino-1,4-cyclohexadiene-1,4-diamine in a low-temperature argon matrix

Isomerization induced by ultraviolet radiation due to intramolecular hydrogen-atom migration in 3,6-diimino-1,4-cyclohexadiene-1,4-diamine (DCD) was studied by matrix-isolation Fourier transform infrared (IR) spectroscopy and density functional theory (DFT) calculation. Two isomers produced by single and double inversion of the hydrogen atom(s) around the C N bond(s) were identified using the wavelength dependence of the IR band-intensity changes. In addition, the IR bands of an intermediate produced by single hydrogen-atom transfer from the amino to the imino group ( C – NH 2 ⋯ NH C ) in the hydrogen bond were measured and identified by a comparison with the calculated spectral patterns. These observations confirm stepwise photoisomerization of DCD.

Infrared spectroscopic and voltammetric study of adsorbed CO on stepped surfaces of copper monocrystalline electrodes

Voltammetric and infrared (IR) spectroscopic measurements were carried out to study adsorbed CO on two series of copper single crystal electrodes n(1 1 1)–(1 1 1) and n(1 1 1)–(1 0 0) in 0.1 M KH 2PO 4 + 0.1 M K 2HPO 4 at 0 °C. Reversible voltammetric waves were observed below −0.55 V versus SHE for adsorption of CO which displaces preadsorbed phosphate anions. The electric charge of the redox waves is proportional to the step atom density for both single crystal series. This fact indicates that phosphate anions are specifically adsorbed on the step sites below −0.55 V versus SHE. Voltammetric measurements indicated that (1 1 1) terrace of Cu is covered with adsorbed CO below −0.5 V versus SHE. Nevertheless, no IR absorption band of adsorbed CO is detected from (1 1 1) terrace. Presence of adsorbed CO on (1 1 1) terrace is presumed which is not visible by the potential difference spectroscopy used in the present work. IR spectroscopic measurements showed that CO is reversibly adsorbed with an on-top manner on copper single crystal electrodes of n(1 1 1)–(1 1 1) and n(1 1 1)–(1 0 0) with approximately same wavenumber of C O stretching vibration of 2070 cm −1. The IR band intensity is proportional to the step atom density. Thus CO is adsorbed on (1 1 1) or (1 0 0) steps on the single crystal surfaces. An analysis of the IR band intensity suggested that one CO molecule is adsorbed on every two or more Cu step atom of the monocrystalline surface. The spectroscopic data were compared with those reported for u h v system. The C O stretching wavenumber of adsorbed CO in the electrode–electrolyte system is 30–40 cm −1 lower than those in u h v system.

Characterization of insoluble organic matter of the Lokpanta oil shales Anambra Basin, by quantitative infrared spectroscopy

Conventional infrared (IR) spectroscopy was used to get quantitative information on the structure of the isolated kerogen of the Lokpanta oil shales in the Anambra basin. The kerogen was extracted from six shale samples collected from different geographical locations in the basin. Only about 1 to 14% of the kerogen present in the fine – grained sedimentary rock extract was recovered from the sedimentary organic residue. Quantitative IR spectroscopy indicates that the kerogen structure consists of about 19% of aliphatic groups ( bands around 2920- 2900, 1450- 1420 cm-1) ; 53% of C=C, C= O, COO- groups (bands around 1610 –1600cm-1) and 28% of aromatic groups (bands around 900 –560 cm-1). These percentages indicate that these kerogens are relatively richer in oxygen bearing functional groups than aliphatic and aromatic groups. This suggest not only that the kerogens are of intermediate maturity but also that they likely correspond to type 11 series, are marine – derived, anoxic, capable of generating oil and gas and originated from the same parent materials. Variation in the intensity of IR bands shows that kerogens from Lokpanta –Lekwesi area are more mature relative to those from Ndeaboh – Awgu. KEY WORDS: Infrared spectroscopy, kerogen, characterization, oil- shales, source –rock, insoluble organic matter, correlation, Lokpanta, Anambra basin. Global Jnl of Pure and Applied Sciences Vol.10(4) 2004: 585-593

Luminescence and EPR studies of defects in Si-SiO2films

Co-sputtered Si-rich SiO 2 films annealed at 1150 °C are studied by photoluminescence and EPR methods. It is found that the emission spectrum of as-prepared samples contains one broad infrared band. It is shown that one-year aging in ambient air and low-temperature annealing in an oxygen atmosphere leads to an increase in the infrared band intensity and the appearance of additional bands with maxima at 1.7 eV, 2.06 eV and 2.3 eV while annealing in a hydrogen atmosphere results in the decrease of the 1.7 eV and 2.06 eV band intensities. A correlation between the 1.7-eV band intensity and the EPR signal from EX-centers is found to exist. The decrease of crystallite sizes results in a high-energy shift of the infrared band while the peak positions of the other ones (at 1.7 eV, 2.06 eV and 2.3 eV) do not change. The infrared band is ascribed to recombination in Si crystallites while the others are attributed to silicon oxide defects. the 1.7-eV and 2.06-eV bands being due to oxygen-excess defects such as EX-center and NBOHC.

Influence of Temperature on the Predictive Ability of near Infrared Spectroscopy Models

Temperature changes alter the position and intensity of near infrared (NIR) spectral absorption bands and thus affect the predictive ability of the associated calibration models. Achieving accurate control of this variable in industrial processes is difficult and variations can have a strong impact on their analytical monitoring. In this work, the effect of temperature changes over the range 25–90°C on the predictions for the ingredients of the esterification reaction between acetic acid and butanol was examined. Spectra for mixtures of the different reactants and products were used to construct calibration models by partial least-squares (PLS) regression and stepwise principal component regression (stepwise PCR). The models were constructed from the temperature ranges, wavelengths, numbers of factors and spectral treatments leading to the highest predictive ability. Based on the results, the variable temperature can also be modelled and the predictive ability of calibration models improved by including partially or completely the effect of temperatures.

Low-temperature anomalies of infrared band intensities and high-pressure behavior of edingtonite

Edingtonite is a ferroelectric material of the Rochelle salt (RS) class and shows large anomalies of different properties in the temperature range of −53 to −133 °C. Classical RS exhibits the low-temperature phase transition P2 12 12– P2 111, whereas ammonium RS shows the phase transition P2 12 12– P1 12 11. According to our data, edingtonite blocks this possible symmetry lowering by the phase transition P2 12 12– P112. Edingtonite, similarly to RS crystals, has two sub-lattices of water molecules and exhibits a complex low-temperature behavior. There are at least three anomalies in the low-temperature dependence of the generation of the second optical 0.53-micron harmonics, excited by the 1.06-micron line of a Nd pulse laser. Edingtonite also exhibits significant anomalies in the low-frequency range of its IR spectrum at low temperatures. The maximum IR anomaly is observed in natural edingtonite at −124 °C, whereas in the deuterated sample the same anomaly occurs at −113 °C. The most intense anomalous broad band at 220 cm −1 did not shift due to deuteration. According to the presented lattice-dynamical calculations, this vibration corresponds to a strong deformation of the cavity around the water molecules during the phase transition. Raman study of edingtonite under high pressure in a diamond anvil cell (DAC) at room temperature shows that there is no pressure-induced phase transition up to 6.4 GPa. No amorphization was observed at this pressure range.

Decay dynamics of the vibrationally activated OH–CO reactant complex

The decay dynamics of the OH–CO reactant complex have been examined following infrared excitation in the OH overtone region using various IR pump–UV probe methods. The time scale for overall decay of the OH–CO (2vOH) complex has been bracketed between 0.19 and 5 ns through linewidth and direct time-domain measurements. The inelastically scattered OH (v=1) fragments exhibit a bimodal internal energy distribution, which reveals that vibrational predissociation proceeds through two pathways. The dominant inelastic decay channel involves vibrational energy transfer from OH to CO with little excess energy remaining for rotational excitation of the OH fragment, while a slower secondary channel releases most of the excess energy as OH rotational excitation. Intermolecular bending excitation of the OH–CO complex through combination bands results in increased rotational excitation of the OH fragments. The most probable OH product states display a strong lambda-doublet preference indicating that the singly occupied pπ orbital of OH is aligned perpendicular to the OH rotation plane following vibrational predissociation of the complex. These product states also minimize the translational recoil of the fragments and maximize the rotational angular momentum of the OH fragment. Abrupt cutoffs in the OH (v=1) fragment internal energy distributions are utilized to determine an upper limit for the ground state binding energy of OH–CO, D0⩽410 cm−1, which is in good accord with ab initio predictions. Finally, a comparison of infrared band intensities obtained using action and depletion detection methods suggests that geared bend and H-atom bend excitation may promote reactive decay of the OH–CO reactant complex.

C−H···O and O−H···O Hydrogen Bonding in Formic Acid Dimer Structures: A QM/MM Study Confirms the Common Origin of Their Different Spectroscopic Behavior

Our previous analysis of the effect of a constant (Onsager reaction) electric field on a formic acid molecule showed that it produced an elongating force on the O−H bond resulting from the parallelism of field and dipole derivative, leading to a red-shifting of the frequency and an increase in infrared band intensity with the field, and a contracting force on the C−H bond resulting from the antiparallelism in the above quantities, leading to a blue-shifting of frequency and an initial decrease, followed by an increase, in band intensity with the field. In this paper we extend this analysis to the characterization of the O−H···O and C−H···O hydrogen bonds in the nonconstant fields present in the formic acid dimer. We use a QM/MM treatment that incorporates exchange-repulsive forces and we also include forces generated by intramolecular cross-term interactions. The excellent agreement with the ab initio bond length changes as a function mainly of the balance of the electrical and repulsive forces demonstrat...

Matrix Infrared Spectra and DFT Calculations of the Reactive MHx (x = 1, 2, and 3), (H2)MH2, MH2+, and MH4- (M = Sc, Y, and La) Species

Laser-ablated Sc, Y, and La react with molecular hydrogen to give MH, MH2+, MH2, MH3, and MH4- (M = Sc, Y, and La) during condensation in excess argon for characterization by matrix infrared spectroscopy. Annealing forms the dihydrogen complex (H2)MH2, which can be reduced to MH4- by electron capture. The (HD)MHD complex exchanges hydrogen positions on broadband photolysis to form primarily the (D2)MH2 complex. Doping the samples with CCl4 to capture ablated electrons markedly increases the MH2+ infrared band intensities and decreases the MH4- absorptions. Further annealing produces higher (H2)2MH2 complexes, which also exchange hydrogen positions. The reaction products are identified by deuterium and deuterium hydride isotopic substitution. DFT and MP2 theoretical calculations are employed to predict geometries and vibrational frequencies of these novel molecules, complexes, anions, and cations. Charged species from laser-ablation contribute more to the spectra of group 3 reaction products than for any o...

Vibrational Spectroscopy of Hydrogen Bonding: Origin of the Different Behavior of the C−H···O Hydrogen Bond

The spectroscopic behavior of the C−H···O hydrogen bond is generally opposite to that of typical (e.g., O−H···O and N−H···O) hydrogen bonds: the C−H stretching frequency is blue-shifted and the infrared band intensity decreases on hydrogen bonding. We show that this can be understood on the basis of the dynamic properties of the donor group, in particular the force on the bond resulting from the interaction of the external electric field created by the acceptor atom with the permanent and induced dipole derivatives of the X−H bond: when the field and dipole derivative are parallel, as in the case of O−H···O, the bond lengthens, and red shift and intensity increases result; when the field and dipole derivative are antiparallel, as in the case of C−H···O, the bond shortens and blue shift results with the possibility of intensity decrease. We demonstrate these properties in an ab initio and perturbation study of cis formic acid in an Onsager reaction field. This analysis provides the basis for a more gener...

Photoinduced rotational isomerization mechanism of 2-chlorobenzaldehyde in low-temperature rare-gas matrices by vibrational and electronic spectroscopies

Rotational isomerization of 2-chlorobenzaldehyde in low-temperature rare-gas matrices has been investigated by vibrational and electronic spectroscopies with aids of the density functional theory (DFT) and configuration interaction single (CIS) calculations. Infrared spectrum of the less stable O-cis isomer, produced from the more stable O-trans isomer upon UV irradiation, is measured with an FT-IR spectrophotometer. The enthalpy difference between the O-cis and O-trans isomers is estimated to be 9.7±0.2kJmol−1 from the temperature dependence of the infrared band intensities. Analyses of the infrared and electronic absorption spectral changes after UV irradiation and the phosphorescence spectra measured at various excitation wavelengths suggest that the rotational isomerization occurs via the intersystem crossing from S1 to T1.

Vibrational and Electronic Spectroscopy of the Fluorene Cation

The fluorene cations, C13H10+ and C13D10+, have been formed by both electron impact ionization and vacuum ultraviolet photoionization, deposited in an argon matrix at 12K and studied via Fourier transform infrared and ultraviolet/visible absorption spectroscopy. Harmonic vibrational frequencies have been calculated using density functional theory (B3LYP/6-31G(d,p)) and vibrational band assignments made. Good agreement is found. Dramatic differences have been observed in the infrared band intensities of the cations compared to their neutral parents. Two excited electronic band systems have been observed and compared with previous photoelectron spectroscopic results. New calculations of the fluorene cation excited states have been performed using configuration interaction singles (CIS), CIS with a doubles correlation correction (CIS(D)), time-dependent Hartree−Fock (TDHF), and time-dependent density functional theory (TDDFT) with SVWN, BLYP, and B3LYP functionals. Theoretically, 12 low-lying excited states ...

Polarized infrared spectroscopic study on the orientation of the molecules in the smectic-C* phase of a ferroelectric liquid crystal with a naphthalene ring: alternative theory for the analysis of polarization-angle-dependent intensity changes.

A theory to explain the polarization-angle dependence of polarized infrared spectra of a ferroelectric liquid crystal in the surface-stabilized ferroelectric liquid crystal state is proposed. It describes the relationship between the intensity of the absorption bands and the polarization angle of the infrared radiation. Using this theory the polarization-angle dependence of the infrared band intensities was analyzed for a ferroelectric liquid crystal with a naphthalene ring and two phenyl rings with a stacked layer structure in the smectic-C* phase. The polarization-angle-dependent spectra were measured at 137 degrees C under external dc electric fields of +40 and -40 V to investigate the orientation of the molecules. Plots of the infrared absorbance versus polarization angle for representative bands were subjected to a curve fitting procedure by a least squares method. From the curves obtained the orientation of the transition dipole moments with respect to the molecular long axis and the orientation of the molecular long axis with respect to the rubbing direction of the cell were estimated based upon the suggested theory. The polarization-angle-dependent infrared spectra obtained were also analyzed by two-dimensional (2D) correlation spectroscopy. The 2D correlation analysis clearly detects a slight phase difference in the polarization-angle dependence which is hardly recognized in ordinary plots of the intensity changes in the infrared bands. The 2D correlation analysis allows us to separate asymmetric and symmetric stretching bands due to the chiral methyl group from those arising from other methyl groups in the alkyl chains.

Infrared and Raman spectra, conformations and ab initio calculations of dichloromethylmethyl dichlorosilane

The infrared (IR) spectra of dichloromethylmethyl dichlorosilane (Cl 2CHCH 3SiCl 2) were recorded in the vapor, amorphous and partly crystalline phases between 4000 and 50 cm −1. IR spectra (4000–450 cm −1) of the compound were also obtained when isolated in argon, nitrogen and krypton matrices at 4.8 K before and after annealing to temperatures in the range 15–50 K. Raman spectra of the liquid were recorded at various temperatures between 295 and 147 K. Spectra of the amorphous and annealed crystal deposited on a copper finger at 78 K were obtained. The spectra showed the existence of two conformers — anti and gauche — in the vapor and in the liquid. Approximately 12 IR bands were reduced in intensity and 6 Raman bands present in the fluid phases vanished upon crystallization. From the intensity variations of two band pairs with temperature, a Δ H 0 value of 0.4±0.3 kJ mol −1 between the conformers was obtained in the liquid, anti being the low-energy conformer. Small increases and decreases in the IR band intensities were observed in the matrix spectra after annealing to 32 (nitrogen), 36 (argon) and 50 K (krypton). The enthalpy difference between the conformers is low in the matrices, but the anti conformer had lower energy than the gauche. Ab initio calculations were performed using the Gaussian-94 program with the RHF/6-311G ∗ basis function and gave optimized geometries, IR and Raman intensities and scaled vibrational frequencies for the anti and gauche conformers. The conformational energy derived was 4.4 kJ mol −1 with anti being the low-energy conformer. The dipole moments were calculated to be 1.2 and 3.1 debye for the anti and gauche conformers, respectively. Correlation between the observed and calculated wavenumbers of both conformers revealed that anti was present in the crystal, and complete assignments of the spectra have been carried out.

PM3, AM1, MNDO and MINDO3 semi-empirical IR spectra simulations for compounds of interest for Titan's chemistry: diazomethane, methyl azide, methyl isocyanide, diacetylene and triacetylene

Four semi-empirical methods (PM3, AM1, MNDO and MINDO3) have been tested to find the best auxiliary tool for the gas chromatography/Fourier transform IR spectroscopy/mass spectrometry (GC/FTIR/MS) identification of five compounds of interest for Titan's atmospheric chemistry as test compounds: diacetylene, triacetylene, diazomethane, methyl azide, methyl isocyanide. Of the four methods, MINDO3 can be considered as the most appropriate method to facilitate the identification of such and similar compounds, since (1) the simulated IR spectra best match the experimental spectra for four compounds of five studied; and (2) MINDO3 provides the best linearity between the calculated and experimental frequencies (correlation coefficient of 0.995; a scaling factor of 0.84 can be applied to afford better correspondence between the calculated and experimental wavenumbers). None of the semi-empirical methods tested is able to predict (even approximately) infrared band intensities, and therefore a spectral intensity pattern.

Preparation and Structural Characterization of Mixed-Stack Charge-Transfer Langmuir−Blodgett Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and N,N,N‘,N‘-Tetramethylbenzidine

One-, three-, and seven-layer Langmuir−Blodgett (LB) films of a mixed-stack charge-transfer (CT) complex of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ) and N,N,N‘,N‘-tetramethylbenzidine (N-TMB) were successfully deposited onto CaF2 plates and gold-evaporated glass slides at the surface pressure of 10 mN m-1. This is the first time that the mixed-stack CT films of N-TMB·C18TCNQ have been prepared by the LB technique. Ultraviolet−visible-near-infrared (UV−vis-NIR) and infrared spectroscopies have been employed to investigate the formation, degree of charge transfer, molecular orientation, and structure of the CT complex in the LB films. The appearance of a broad absorption band with its center at about 1750 nm reveals that the CT complex in the LB films is in the mixed-stack form. The frequency of a C⋮N stretching band of C18TCNQ in the CT complex indicates that the degree of charge transfer (ρ) is about 0.24. A comparison of the infrared band intensities between the transmission and RA spectra ...

New conjugated polyenes with 1,3-dialkyl-2-thiobarbituric acid moiety as materials for nonlinear optics: theoretical calculations, synthesis and spectral properties

In this work we report the results of our study on electronic and spectral properties of conjugated polyenes with electron-accepting 1,3-dialkyl-2-thiobarbituric acid moiety. In model calculations, we examine the effect of the conjugated polyene length on infrared (IR) and Raman spectra of the polyenes by means of ab initio HF/3-21G*. Nonlinear properties were also studied by AM1 method in frames of the sum-over-states (SOS) and finite-field formalism. It was concluded that in well-resolved IR and Raman spectra the frequencies and band intensities can provide valuable information relating to CC bond lengths in polyene chain and relative polarizabilities. Near-linear correlation between polarizability and integral IR band intensity, corresponding to all CC stretching modes, and the rather nonlinear relationship of polarizability with integral Raman activity, was found. In our calculation we predict that polarizability and the first hyperpolarizability increases with elongation of polyene chain while the second hyperpolarizability increases smoothly in a quadratic way. In contrast to the linear relationship between polarizability and polyene chain length the dipole moment versus chain length is predicted to be nonlinear. A good agreement was found between experimental and calculated Raman spectra of one newly synthesized compound studied.

Near-infrared study of short-range disorder of OH and F in monoclinic amphiboles

Amphiboles were synthesized along the joins tremolite-fluorotremolite, richterite-fluororichterite and potassic-richterite-potassic-fluororichterite at 750 °C and 1 kbar P(H 2 O). Infrared spectra of the amphiboles were recorded in the principal OH-stretching region. Amphiboles of the tremolite-fluorotremolite series show one-mode behavior, a single band due to a local MgMgMg-OH- A ⃞ (⃞ = vacancy) arrangement; this behavior is consistent with no coupling between NNN (next-nearest-neighbor) O3 anions either through the O3- O3 edge or across the vacant A-site cavity. The amphiboles of the richterite-fluororichterite and potassic-richterite-potassic-fluororichterite series show two-mode behavior, two bands due to the local arrangements MgMgMg-OH- A Na-OH and MgMgMg-OH- A Na-F (and their K equivalents); this behavior is consistent with coupling between NNN O3 anions across the filled A-site cavity through Na or K that occupies the A-site. A mathematical model is developed to describe local (OH,F) ordering in amphiboles as a function of F content. The variation in infrared band intensities is consistent with complete short-range disorder of OH and F in the synthetic amphiboles of the richterite-fluororichterite and potassicrichterite- potassic-fluororichterite series.