Absolute Band Intensities Research Articles

An analytical expression for temperature-dependent infrared intensities in fully coupled anharmonic molecular systems

We present the analytical expression for determining the temperature evolution of absolute integrated infrared (IR) intensities of fundamental bands in fully coupled anharmonic molecular systems. Only harmonic frequencies and integrated IR intensities (within the double harmonic approximation) are needed to predict the absolute temperature-dependent IR intensities of the fundamental bands. These quantities can be readily obtained from ab initio electronic structure calculations for relatively large molecules. In order to illustrate the accuracy of the expression we compare the results from this formula using harmonic frequencies and intensities from a tight-binding potential energy surface with the results from semiclassically prepared molecular dynamics (SCP-MD) simulations on the same potential energy surface for the naphthalene molecule. For a favorable comparison with experimental data we use the same analytical expression with the more accurate B3LYP/cc-pVDZ harmonic frequencies and intensities. We conclude that the IR intensity changes that our proposed formula predicts in the fundamental vibrational bands are mainly due to intramolecular vibrational energy redistribution (IVR).

Towards the intensity consistency of the ozone bands in the infrared range: Ab initio corrections to the S&MPO database

Accurate knowledge of line parameters, and particularly line intensities is of primary importance for satellite, ground based and balloon measurements of the ozone concentration in the atmosphere. Self-consistency in absolute band intensities of ozone in various spectral intervals is necessary to avoid discrepancies in the atmospheric ozone retrieval from different windows. Difficulties in precise experimental characterization of absolute line intensities of this unstable molecule are well known leading to many significant deviations in published intensity values and in spectroscopic compilations. Ab initio intensities computed from the dipole moment functions of our previous works (Tyuterev et al. JCP 2017;146:064304; JCP 2019;150:184303) have permitted achieving a consistency between Stark-effect measurements and various databases in 10 and 5 µm in a good agreement with the most accurate recent experiments. In this paper we extend ab initio corrections to available line lists of ozone band intensities in wider IR range up to the bands with four vibrational quanta. About 40% of intensities were corrected in the 0 - 4122 cm−1 range for thirty cold and hot bands of the main ozone 16O3 isotopologue. Most of them, but not all, were obtained using band specific scaling factors ranging from 0.75 to 1.17 making quite significant changes in the absolute values. The full line lists containing corrected line intensities in the spectral intervals considered in the paper are provided in the Supplementary material for this work. The resulting line list is now available on the web site of the “Spectroscopy and Molecular Properties of Ozone” (S&MPO) information system (http://smpo.iao.ru; http://smpo.univ-reims.fr). A large part of this line list including the ab initio correction reported in this work will be used in the future HITRAN20 update.

To the choice of spectral region for laser based analysis of molecular gases at strong interference of absorption spectra

The interference of the absorption bands of H2O, CO2, and several gaseous biomarkers was analyzed in the middle and near-IR spectral region using the HITRAN2006 database. It was demonstrated that the problem o f the vibration-r o tation band interfere nce for detected and interfering gases is aggravated in the near-IR spectral region. This aggravation is caused not only by the decrease in the absolute band intensity for overtones and combination bands of the molecular vibrations but also by the relative increase in the absorption in H2O lines and high density of CO2 lines in the near-IR spectral region

Calculation and Comparative Analysis of Ir Spectra of Homobrassinolide and (22S,23S)-24-Epibrassinolide

Normal vibrational frequencies and absolute IR band intensities of the biologically active steroid phytohormones homobrassinolide and (22S,23S)-homobrassinolide were calculated in the framework of an original approach that combined classical analysis of normal modes using molecular mechanics with quantum-chemical estimation of the absolute intensities. IR absorption bands were interpreted based on a comparison of the experimental and theoretical absorption spectra. The impact of structural differences in the side chains of these molecules on the formation of their IR spectra in the region 1500–950 cm −1 was estimated.

FTIR and dispersive gas phase absolute infrared intensities of hydrocarbon fundamental bands.

New experimental intensity results obtained by band integration from the PNNL (Pacific Northwest National Laboratory) spectral library are reported for 26 CH vibrations of methane, acetylene, ethylene, ethane, allene, propyne and cyclopropane. The PNNL intensity values range from 3.1 to 185.4 km mol-1 and are in excellent agreement, rms difference of 3.1 km mol-1, with earlier low resolution intensity results. QCISD/6-311++G(3d,3p) and QCISD/cc-pVTZ theoretical results are in good agreement with the PNNL intensity values with rms differences of 4.4 and 4.9 km mol-1, respectively. Charge-charge transfer-dipolar polarization model parameters at both quantum levels indicate that the charge transfer-dipolar polarization contributions to the intensities are much larger than those owing to the movements of static equilibrium charges on hydrogen for these vibrations except for the CH vibrations of acetylene that is known to contain very acidic hydrogen atoms. The main effect of the static charge movement comes from its interaction with charge transfer-dipolar polarization owing to the relatively large parameter values of this dynamic electronic contribution. The sum of the charge transfer-dipolar polarization parameters with their interaction with the charge accurately describes the variations in the intensity values of these hydrocarbon vibrations.

UV absorption of Criegee intermediates: quantitative cross sections from high-level ab initio theory.

Criegee intermediates (CIs) are increasingly recognized as important intermediates in atmospheric and combustion chemistry. Modelling in these fields requires reliable characterization of the CI's UV absorption cross sections. Different experimental approaches provided seemingly contradictory results both for the simplest CI as well as for higher analogues. To resolve these discrepancies, we model simultaneously the positions, widths, shapes and absolute intensities of the first absorption bands of CIs using the semiclassical reflection principle approach based on the path integral molecular dynamics. The UV spectra were efficiently processed via the kernel density estimation method. We show that the multireference character complicates the description of the system and the appropriate treatment of the electron correlation is vital. Multi-reference methods with dynamical correlation are problematic due to high sensitivity of the results with respect to the active space. Single-reference based methods become reliable once the triple excitations components are included. Interestingly, the very similar CC2 and ADC(2) methods provide highly conflicting results which are, however, reconciled at the CC3 and ADC(3) levels. To calculate the spectra of higher CIs, we introduce composite EOM-CCSD/T and CC2/3 methods providing excellent absorption cross sections at an acceptable computational cost. We provide the first absolute intensities for individual conformers of the CH3CHOO molecule.

Kerogen thermal maturity and content of organic-rich mudrocks determined using stochastic linear regression models applied to diffuse reflectance IR Fourier transform spectroscopy (DRIFTS)

A method is presented to estimate thermal maturity and concentration of sedimentary organic matter (kerogen) in organic-rich mudrocks using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The maturity estimate is based on stochastic regression model built on a diverse set of mudrocks containing type II kerogen and spanning a range of age, lithology, kerogen content, and maturity. The model accounts for variations in intensities of both aliphatic and aromatic CH absorption bands. It has been validated on over forty mudrock samples with vitrinite reflectance values from ∼0.3 to 3%Ro. The maturity estimate is accurate on average to better than 0.2%Ro up to vitrinite reflectance values of at least 3%Ro, on par with uncertainties associated with vitrinite reflectance measurements.Estimating the kerogen content of mudrocks using infrared (IR) spectroscopy is challenging because the absolute intensities of aliphatic and aromatic CH absorption bands in infrared spectra are influenced by both kerogen concentration and composition, and composition changes as a function of maturity and type. A scaling factor for type II kerogen standard spectra has been derived as a function of maturity, such that the maturity estimate from the model is used to apply the scaling necessary to estimate kerogen content. The resulting kerogen concentrations agree with reference concentrations within about 1.5wt% for marine mudrocks. Thus, this approach provides a rapid and integrated analysis of kerogen maturity and content, in addition to established mineral analysis.

ChemInform Abstract: Raman and Infra‐Red Microspectroscopy: Towards Quantitative Evaluation for Clinical Research by Ratiometric Analysis

AbstractReview: 123 refs.

Excited-State Proton Transfer Can Tune the Color of Protein Fluorescent Markers.

We show by quantum mechanical/molecular mechanical (QM/MM) simulations that phenylbenzothiazoles undergoing an excited-state proton transfer (ESPT) can be used to probe protein binding sites. For 2-(2'-hydroxy-4'-aminophenyl)benzothiazole (HABT) bound to a tyrosine kinase, the absolute and relative intensities of the fluorescence bands arising from the enol and keto forms are found to be strongly dependent on the active-site conformation. The emission properties are tuned by hydrogen-bonding interactions of HABT with the neighboring amino acid T766 and with active-site water. The use of ESPT tuners opens the possibility of creating two-color fluorescent markers for protein binding sites, with potential applications in the detection of mutations in cancer cell lines.

Quantitative vapor-phase IR intensities and DFT computations to predict absolute IR spectra based on molecular structure: I. Alkanes

Recently recorded quantitative IR spectra of a variety of gas-phase alkanes are shown to have integrated intensities in both the C3H stretching and C3H bending regions that depend linearly on the molecular size, i.e. the number of C3H bonds. This result is well predicted from CH4 to C15H32 by density functional theory (DFT) computations of IR spectra using Becke's three parameter functional (B3LYP/6–31+G(d,p)). Using the experimental data, a simple model predicting the absolute IR band intensities of alkanes based only on structural formula is proposed: For the C3H stretching band envelope centered near 2930cm−1 this is given by (km/mol) CH_str=(34±1)×CH−(41±23) where CH is number of C3H bonds in the alkane. The linearity is explained in terms of coordinated motion of methylene groups rather than the summed intensities of autonomous –CH2-units. The effect of alkyl chain length on the intensity of a C3H bending mode is explored and interpreted in terms of conformer distribution. The relative intensity contribution of a methyl mode compared to the total C3H stretch intensity is shown to be linear in the number of methyl groups in the alkane, and can be used to predict quantitative spectra a priori based on structure alone.

Evaluation of the electrooptical parameter system in the valence-optical scheme

A combined method for solving the inverse electrooptical problem was considered using as examples 36 organic compounds for which the absolute band intensities in their IR absorption spectra were known. Data from quantum-chemical calculations were used to find the initial values of the electrooptical parameters; experimental data for the absolute band intensities, to determine the final values. It was shown that the previously observed locality of the parameters and their transfer into a series of related molecules were confirmed by direct calculations.

Fourier transform spectroscopy using a synchrotron source: Characterization of the instrument line shape and acetylene line parameters around 100 μm

The acetylene 100μm spectral region of astrophysical interest was studied to retrieve absolute line positions and intensities of weak hot bands. An absorption path of 150m was used and the spectra were recorded using the far-infrared AILES beamline of the synchrotron SOLEIL and a Bruker IFS 125-HR. To achieve accurate measurements from FT spectra using the full beam of the synchrotron, special cares have been taken into account concerning the apparatus function and especially the optical weighting. The wavenumber dependence of the size of the synchrotron beam was modeled leading to similar results that have been previously obtained for the analysis of the ν5–ν4 band ([8] Jacquemart et al., J Quant Spectrosc Radiat Transfer 2010;111:1223–33) but with a better precision. In the present work, line positions and intensities have been measured for the first time for 816 transitions of 12C2H2 belonging to 17 bands between 51 and 215cm−1. The accuracy of measurements are estimated to be around 0.1×10−3cm−1 for line positions and between 5 and 10% for intensities depending on the strength of the transitions. The spectroscopic knowledge of these hot bands is of importance especially for theoretical and astrophysical applications.

High sensitivity CW-Cavity Ring Down Spectroscopy of N2O between 6950 and 7653 cm−1 (1.44–1.31 μm): II. Line intensities

The room temperature absorption spectra of nitrous oxide, N2O, have been recorded in the 6950–7653 cm−1 spectral region at 2 and 10Torr using a CW-CRDS spectrometer based on 24 fibered DFB lasers. The achieved sensitivity (noise equivalent absorption αmin≈5×10–11cm−1) allowed detecting lines with intensities as low as 1×10–29cm/molecule. In the preceding contribution (Lu Y, Mondelain D, Liu AW, Perevalov VI, Kassi S, Campargue A, J Quant Spectros and Radiat Transfer 2012;113:749–62), we reported the assignment of more than 7200 N2O lines in the region and the derivation of the corresponding spectroscopic parameters Gv, Bv, Dv and Hv. In the present work, more than 1300 14N216O absolute line intensities of cold and hot bands belonging to the ΔP=12, 13 and 14 series of transitions have been measured (P=2V1+V2+4V3 is the polyad number). The uncertainty of the obtained line intensity values varies from 4 to 7% for the majority of the lines. The obtained dataset extends importantly the set of measurements available in the literature, in particular for the ΔP =13 series for which previous data were very limited. The ΔP=12–14 effective dipole moment parameters were fitted to the intensity values measured in this work and available in the literature. The obtained sets of the dipole moment parameters allow reproducing the observed line intensities within their experimental uncertainties. The calculated intensities of the ΔP=12, 13 and 14 bands of 14N216O assigned by CRDS in the 6950–7653 cm−1 region are provided as supplementary material.

On the accuracy of ab initio calculations of absolute band intensities in the IR spectra of molecules

Absolute intensities are calculated in the harmonic approximation for the IR spectrum of 18 hydrocarbons, oxygen- and nitrogen-containing organic compounds. The quantum chemical calculation is carried out in the 6-311G(3df,3pd) basis set. The calculated data are compared to the experimental values of absolute absorption intensities. It is found that calculations in the HF approximation substantially overestimate (on average by 87%) the integral absolute intensity of fundamental vibrations in the 575–4000 cm−1 range. Most part of this overestimation falls on the stretching vibrations with large amplitudes (C-H and C=O bonds). When the MP2 method is used without electron correlation treatment, this overestimation decreases to 22% and becomes more uniform in the whole spectral region.

Ab initio calculation of band absolute intensities in IR spectra

Ab initio calculations in a harmonic approximation of absorption band absolute intensities in infrared spectra were carried out for 3 hydrocarbons and 14 halogenated hydrocarbons. The calculated data were compared with experimental values of the absolute absorption intensities. It is shown that a Hartree–Fock calculation method overestimates significantly (by an average of 66%) the integrated absolute intensities of the fundamental bands in the region 575–4000 cm–1. The deviation is reduced to 32% in the case of the MP2 method of accounting for electron correlations. Most of the overestimation occurs for bands corresponding to vibrations involving halogen atoms.

Applicability of the quantum-chemical calculations of potential and electrooptical function parameters to the standardless spectrum analysis of heteroatomic organic compounds

The quantum-chemical calculation of potential and electrooptical function parameters for 32 heteroatomic (nitrogen-, oxygen-, and sulfur-containing) compounds was performed with the use of the B3LYP/6-311G(3df,3pd) basis set. With the use of a small correction of the potential function with unified scaling factors, the calculated frequencies and absolute intensities of fundamental absorption bands agreed satisfactorily with the experimental values. The average and standard deviations of calculated and experimental frequencies were no higher than the half widths of their absorption bands. The results suggest the applicability of these calculations to the standardless spectrum analysis of gas mixtures.

Spectroscopic characteristics of dimethylsulfoxide molecules coordinated to Mg2+ cation. structure of complexes [Mg(DMSO) i (CH3CN)6-i ]2+ from the data of quantum-chemical calculations and IR spectra of Mg(ClO4)2-DMSO-CH3CN Solutions

By the DFT B3LYP/6-31G** method the geometry was optimized and IR spectra were calculated of complexes [Mg(DMSO)i(CH3CN)6-i]2+ (i = 1–6). The values of free energy ΔG in the reaction of ligands substitution in the coordination sphere of the cation were determined. A satisfactory agreement between experimental and calculated values of structural parameters and infrared spectra of free molecules and coordinated to the cation DMSO was obtained. The regularities in the changes of the spectroscopic and structural characteristics of [Mg(DMSO)i(CH3CN)6-i]2+ complexes at varying their composition were revealed. The frequencies and absolute integral intensities of DMSO bands in the IR spectra of pure liquid, solutions in acetonitrile, and in three-component solutions Mg(ClO4)2-DMSO-CH3CN were measured. A correspondence between the calculated change of the frequency and absolute intensity of the IR bands v(C≡N), v(C-C), v(S=O), and v(SC) of the complexes and the corresponding values in the IR spectra of the solutions with different content of components of binary solvent was found.

FT-IR Spectroscopy and DFT Calculations on Fluorinated Macromer Diols: IR Intensity and Association Properties

Fluorinated macromeric diols have been investigated by means of Fourier transform infrared (FT-IR) spectroscopy; the absolute intensities of characteristic bands of OH stretching have been determined and related to aggregation phenomena. Main results show that the ratio between "free" and "hydrogen-bonded" OH groups strictly depends on the polarity of chemical environment (macromeric-polymeric matrix). The intepretation of the experimental data has been supported by density functional theory (DFT) calculations on suitable molecular models, validating the results obtained both qualitatively and quantitatively.

Calculation of infrared spectra for conformers of a dimeric fragment of the 2,6-hydroxyethylcellulose macromolecule in aqueous solution

Based on both a complete calculation of normal vibration frequencies and absolute IR absorption band inten- sities of a dimeric fragment of the 2,6-hydroxyethylcellulose macromolecule and a comparison of the results with the corresponding experimental data, a detailed interpretation of the IR spectrum (1200-960 cm -1 ) of this polymer is given for the first time. Its conformational characteristics in aqueous solution at 20 and 70 o C were determined by the molecular dynamics method. It is shown that the changes observed in IR spectra of hydroxyethylcellulose in aqueous solutions upon heating may be connected with conformational transitions of the ether side groups. analysis, absolute intensities, spectrum interpretation. Introduction. Water-soluble ethers of cellulose (hydroxyethyl-, carboxymethyl-, methyl-derivatives) are widely used in various industrial sectors. Structural features, in particular, the number and location of ether groups on the pyranose ring and their rotational isomerization are responsible for the valuable properties of these compounds. Vibra- tional spectra (IR and Raman) can reveal even the most subtle structural changes in these macromolcules. Therefore, they are exceedingly effective for studying the structural organization of water-soluble ethers of cellulose (1-4). The reversible changes in the spectra that occur on changing the temperature of the polymers are not related to any chemi- cal transformations in the structure of these compounds and can be explained (5) by a change of the rotational iso- meric (conformational) structure of the macromolecules (6, 7). Studies of the conformational features of the side groups in water-soluble ethers of polysaccharides should establish structureproperty relationships for these practically important compounds. The goal of the present work was to determine the structure of side groups in a dimeric fragment of the water-soluble cellulose ether macromolecule 2,6-hydroxyethylcellulose (2,6-HEC) in aqueous solution at various tem- peratures, to calculate the frequencies and potential energy distribution (PED) of normal vibrations and absolute inten- sities of IR absorption bands, and to model the optical density spectrum of the most stable conformers of the ether groups of this fragment. Experimental Method. The stated problem was solved using a combined approach (8) to modeling IR spec- tra of organic compounds that used both molecular mechanics and quantum chemistry methods to calculate the fre- quencies of normal vibrations and absolute intensities of IR absorption bands, respectively, of polyatomic molecules. The algorithm for calculating the absolute intensities within the framework of the applied approach (as for using the valence force field approximation (8, 9)) consisted of calculating the derivatives of the electron-density P matrix over Decartes shifts of atoms and the matrix of derivatives ∂r ⁄ ∂Qi from Decartes shifts of atoms over normal coordinates and multiplying the resulting matrices. Shifts of atoms in Decartes coordinates in each normal vibration were calcu- lated using molecular mechanics (solution of the direct spectral problem). Matrices of charges on atomic nuclei, elec- tron densities, and bond orders were calculated using the CNDO/2 quantum-chemical method. The derivatives of these

C2H2 Overtones Near 12300 cm1 Revisited with a Very Sensitive Cavity Ring-down Spectrometer

A cavity ring-down spectrometer (CRDS) is constructed with a single-mode continuous-wave Ti: Sapphire laser. It allows attaining a minimum detectable absorption of 1.8 1010 cm1. The spectrometer is applied to record the overtone spectrum of 12C2H2 in the 1224012350 cm1. Compared with the previous CRDS and intra-cavity laser absorption spectroscopy studies in the same region, the present measurement achieved better sensitivity and better precision as well. As a result, the ro-vibrational parameters of the high overtone bands of acetylene at 12290.12, 12311.82, and 12350.61 cm1 have been refined. The advantages of the present CRD spectrometer is also demonstrated by the newly observed and well characterized perturbation on the f component of the very weak band near 12289 cm1. The quantitative measurement capability of the spectrometer is verified with the measurement of the water lines and employed to give the absolute band intensities of those three acetylene bands.