Absorption Spectra Of Polyatomic Molecules Research Articles

Greenhouse effect in planetary atmospheres caused by molecular symmetry breaking in intermolecular interactions

It is believed that the greenhouse effect is related to the parameters of absorption spectra of polyatomic molecules, usually trace gases, in planetary atmospheres. The main components of all known atmospheres of celestial bodies are symmetrical molecules that do not possess the dipole-allowed purely rotational (and in the case of diatomic molecules, vibrational–rotational) absorption spectrum. Upon increased pressure, a weak absorption appears, induced by intermolecular interaction, which can lead to a greenhouse effect. The contribution of the induced absorption in radiative forcing of a dense atmosphere may amount to a few or even tens of W/m2. In conditions typical for the atmospheres of terrestrial planets (including paleoatmospheres), the collision-induced absorption and associated greenhouse effect may lead to an increase in surface temperature above the freezing point of water. There is a correlation between the temperature of an atmosphere and the intermolecular bonding energy of gases that dominate in planetary atmospheres of the Solar System.

On the structure of hot absorption spectra of polyatomic molecules: Solvent effect on the transition energy gap

Hot absorption spectra of polyatomic molecules may exhibit a characteristic shoulder. Its origin and connection to the 0-0 transition energy gap has been established on the basis of a recently developed quantum-classical approach. We demonstrate that an accurate estimate of the transition energy can be obtained directly from experimental data. The method can be used to study the solvent influence on the energetics of electronic transitions.

First principles simulation of the UV absorption spectrum of ethylene using the vertical Franck-Condon approach.

A new method which we refer to as vertical Franck-Condon is proposed to calculate electronic absorption spectra of polyatomic molecules. In accord with the short-time picture of spectroscopy, the excited-state potential energy surface is expanded at the ground-state equilibrium geometry and the focus of the approach is more on the overall shape of the spectrum and the positions of the band maxima, rather than the precise position of the 0-0 lines. The Born-Oppenheimer approximation and the separability of the excited-state potential energy surface along the excited-state normal mode coordinates are assumed. However, the potential surface is not necessarily approximated as harmonic oscillator potentials along the individual normal modes. Instead, depending upon the nature of the potential surface along a particular normal mode, it is treated either in the harmonic approximation or the full one-dimensional potential is considered along this mode. The vertical Franck-Condon approach is applicable therefore even in cases where the excited state potential energy surface is highly anharmonic and the conventional harmonic Franck-Condon approach is inadequate. As an application of the method, the ultraviolet spectrum of ethylene between 6.2 eV (50,000 cm(-1)) and 8.7 eV (70,000 cm(-1)) is simulated, using the Similarity Transformed Equation of Motion Coupled-Cluster method to describe the required features of the potential energy surfaces. The spectrum is shown to be a result of sharp doublet structures stemming from the pi --> 3s (Rydberg) state superimposed on top of a broad band resulting from the pi --> pi* (valence) state. For the Rydberg state, the symmetric C=C stretch and the torsion mode contribute to the spectrum, while the broad valence band results from excitation into the C=C stretch, CH2 scissors, and the torsion mode. For both states, the potential along the torsion mode is highly anharmonic and the full treatment of the potential along this mode in the vertical Franck-Condon method is required.

Quantum-mechanical description of the intensity distribution in two-photon absorption spectra of dihalogenated benzenes

Using molecules of dihalogenated benzenes as an example, the verification of an earlier proposed method of direct quantum-mechanical calculation of the relative intensities of lines in the two-photon absorption spectra of polyatomic molecules is continued. The method of calculation is based on the adiabatic model in the context of the Herzberg-Teller approximation. A comparative analysis of the intensity distribution in the two-photon absorption spectra of dihalogenated and monosubstituted benzenes is carried out, and the vibrational characteristics of these molecules in the lowest singlet excited electronic states are determined. A satisfactory agreement with experimental values, along with the analysis of the possibilities of the method, confirms the expediency of its use for gaining information about the intensity distribution in two-photon absorption spectra of polyatomic molecules.

Luminescence and absorption spectra of polyatomic molecules subjected to conformational transitions

A general approach and results of specific calculations of conjugated vibronic absorption and fluorescence spectra of “soft” molecules subjected to conformational transitions are presented. The vibronic spectra of these molecules exhibit significant anomalies such as a large energy gap between the fluorescence and absorption spectra, a strong deviation from the mirror symmetry, etc., which cannot be explained within the framework of the theory of vibronic spectra based on single-well adiabatic potentials.

The quality of ab initio quantum mechanical prediction of vibrational transition dipole directions and infrared intensities

This is a preliminary report of our research aimed at the joint use of up-to-date experimental (polarized infrared spectra of oriented samples) and theoretical (ab initio quantum chemical) methods of determining the vibrational electric transition moment directions to help in the assignment of infrared absorption spectra of polyatomic molecules. The systems studied include ethylene, p-nitrobromobenzene, thiosemicarbazide and N, N-dimethyl- S-methylmonothiobarbamate. Our results show that this bilateral approach works well for molecules with orthogonal ( C 2 ν or higher) symmetry. Special emphasis was given to testing the capabilities of the method for systems with lower symmetry ( C 2h, C s, etc.) where it proved to be no more successful than the prediction of infrared intensities: agreement between the calculated and measured band polarization values within tight limits is quite rare, while completely wrong results have also been obtained, especially in the presence of unaccounted hydrogen bonding.

Calculation of intensity in the resonance Raman and two-photon absorption spectra of polyatomic molecules

A direct method for calculating the resonance Raman and two-photon absorption spectra of polyatomic molecules is described in detail The method is based on the adiabatic model and uses Herzberg-Teller’s approximation. Relations ruling out direct summation over vibrational quantum numbers of excited electronic states and representing the matrix elements of the Green function of a multidimensional oscillator as functions of vibration frequencies and Dushinsky transformation parameters are derived. The relations are convenient for constructing algorithms.

Evolution strategies applied to least-squares curve fitting of spectroscopic data

A robust method for nonlinear least-squares curve fitting without using function derivatives is presented. An advanced evolution strategy is applied to the fitting of high-resolution absorption spectra of polyatomic molecules to estimate half-widths and broadening coefficients. A comparison with the Levenberg-Marquardt algorithm shows that the evolution strategy yields the same results while being less sensitive to the choice of the initial parameters, particularly in the case of crowded spectra.

Investigation of broad molecular absorption bands with the help of line shape analysis in coherent Raman scattering experiments

AbstractThe line shape observed in coherent Raman scattering (CRS) with excitation under conditions of electronic resonance is discussed, For the case of A‐type scattering, the complex Raman amplitude is related directly to the molecular one‐photon absorbance of the resonance transition concerned. The basis for this relation is transform theory, a method known from spontaneous resonance Raman scattering to relate the Raman excitation profile to the observed molecular absorbance. Features of the Raman amplitude, determining the observed line shapes for coherent anti‐Stokes Raman scattering (CARS) and for coherent Stokes Raman scattering (CSRS), are discussed qualitatively. The discussion considers two typical forms in which broad molecular absorption spectra of polyatomic molecules in the condensed phases appear, Inhomogeneous broadening leads to characteristic changes of the Raman amplitude in relation to molecular absorbance in the case of CSRS (formolecules in the electronic ground state), whereas for CARS the relation derived in the homogeneous broadened case remains valid. The special advantages of the CRS technique for determining the complex non‐Raman resonant (background) susceptibility from concentration dependence are demonstrated.

High Temperature Ultraviolet Absorption Spectra of Polyatomic Molecules in Shock Waves

AbstractThe ultraviolet absorption spectra of substituted cycloheptatrienes, toluene, p‐xylene, and of benzyl and methyl‐benzyl radicals have been measured in shock waves at temperatures between 700 and 1650 K. The temperature dependence of the spectra is compared with an analysis based on the deltafunction method in various approximations. The measurements allow for the prediction of the shape of microcanonically hot spectra of laser excited molecules.

Some probiems op additivity in the IR absorption spectra

The problem of additivity of molecular dipole moment; is discussed. The mathematical expressions of additivity concept are derived for various parametric theories of intensities in the IR absorption spectra of polyatomic molecules. New criteria of dipole moment additivity are suggested in terms of Morcillo's and Mayants-Ayerbukh's formalism. The necessary and sufficient condition of transferability of intensity theory parameters is considered as well.

Diffuseness of the vibrational absorption spectra of polyatomic molecules due to sequence congestion of the vibrational levels

Diffuseness of the vibrational absorption spectra of polyatomic molecules due to sequence congestion of the vibrational levels

Electronic relaxation as a cause of diffuseness in electronic spectra

Diffuseness, as seen in the vapour absorption spectra of polyatomic molecules, is related with rapid electronic relaxation (radiationless transitions). The causes common to both are perturbations between electronic states. Line broadening is related to excited state lifetimes through the uncertainty principle. To this end, twelve causes of diffuseness are first listed, and their incidence examined. A distinction is drawn between intrinsic diffuseness, associated with excitation to or perturbation by a (quasi-)continuum and including the electronic relaxation mechanism here proposed, and trivial diffuseness due to overcrowding of individually sharp lines. Trivial diffuseness may be temperature dependent (structureless rotational envelopes; overcrowded sequence bands) or, in very large molecules, temperature independent. I t has been shown that rotational envelopes should contain sharp features in a number of transitions which prove, experimentally, to be diffuse. Because of sequence bands, however, there is for any polyatomic molecule a fairly well-defined critical temperature, calculable from ground state vibrational data, above which the entire electronic spectrum must become structureless. This critical temperature decreases with increasing molecular size and with increasing molecular flexibility, and in conjunction with vapour pressure considerations sets definable limits to the kinds of molecule for which structured spectra can be obtained in the gas phase. But this mechanism is insufficient to explain the frequent occurrence of structureless spectra in small rigid molecules. The literature on medium to high resolution spectra of polyatomic molecules is then reviewed, to establish the generalization that the only transitions with structure (if any) will normally be the first triplet (if observable) and the first singlet, and possibly (for special reasons) Rydberg transitions. This survey is complemented by measurements on the following molecules: pyridine N-oxide, indene, indazole, purine, quinoline, isoquinoline, 1,6-naphthyridine, quinazoline, quinoxaline, 1,4,5- triazanaphthalene, biphenylene, fluorene, acridine, tetramethylcyclobutane-1,3- dione, and all monoalkylbenzenes from ethyl to t-butyl. The proposed generalization is found to have wide validity, though there are one or two undeniable exceptions and a few marginal cases. It is associated with Kasha's rule concerning states capable of emitting radiation, though the correlation between the two is not necessarily one-to- one. Taken together, these rules and the evidence on which they are based leave the electronic relaxation mechanism as the only satisfactory explanation so far offered for the widespread occurrence of diffuse electronic transitions in polyatomics. This matter can also be studied in pure and mixed crystals, if it is assumed, as now seems very probable, that the crystalline matrix induces no line-narrowing. Recent systematic work on line widths in crystal spectra a t 4°K by Hochstrasser and Marzzaccol is in full accord with the viewpoint advanced here.

Интенсивности в инфракрасных спектрах поглощения многоатомных молекул

Интенсивности в инфракрасных спектрах поглощения многоатомных молекул

A general method for the localization of electronic transition frequencies in continuous absorption spectra.

THE more or less complete lack of vibrational structure in ultra-violet absorption spectra of polyatomic molecules has made it impossible to locate the frequency corresponding to the pure electronic transition. This has further been an obstacle to the semi-empirical development of quantum mechanical theories concerning the electronic structure of complex molecules. In a short communication1 we have reported some experiments dealing with the temperature effect on the absorption spectrum of cyclo-octatetraene. In this connexion we directed attention to a possible method of locating the electronic frequency in continuous absorption spectra.

A Comparison of Some Ultra-violet Absorption Spectra of Polyatomic Molecules with Those of Diatomic Molecules.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTA Comparison of Some Ultra-violet Absorption Spectra of Polyatomic Molecules with Those of Diatomic Molecules.W. Albert. NoyesCite this: J. Phys. Chem. 1937, 41, 1, 81–89Publication Date (Print):January 1, 1937Publication History Published online1 May 2002Published inissue 1 January 1937https://doi.org/10.1021/j150379a007RIGHTS & PERMISSIONSArticle Views34Altmetric-Citations1LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (559 KB) Get e-Alerts

The absorption spectra of polyatomic molecules containing methyl and ethyl radicals—III

The study of the absorption spectra of simple polyatomic molecules has many objects. As for diatomic molecules, it may lead to a deeper understanding of the interpretation of the spectra of polyatomic molecules in terms of spectral theory. As a consequence it may also lead to chemical data such as the energies of linkage, interatomic distances, and the electronic configurations of the molecules. In addition, it may help to explain the mechanism and nature of the primary processes in photochemical decomposition. In passing from diatomic to polyatomic molecules, however, two additional matters become of importance: on the one hand, the nature and types of vibration (valency or deformation, symmetrical or antisymmetrical) induced by the electronic excitation are of interest, and, on the other, it is desirable to examine how far symmetry properties assumed on the basis of independently determined molecular structures are borne out in the structure of their spectra. The difficulties in these studies are not solely due to the often very complicated structure of polyatomic molecule spectra, but also frequently to the continuous nature of these spectra in the ultra-violet region. It is with this region of absorption that photochemical considerations are usually concerned, the frequencies associated with it corresponding to transitions between the lower electronic states of the molecules. The interpretation of the continua is at the present time far from clear. In some cases work at low pressures reveals the existence of bands which so merge together with increasing pressure as to give effectively continuous absorption, whilst in other cases where the continua are apparently genuine, the influence of pressure upon the long wave limit of absorption is considerable. Even the conclusions to be drawn from the diffuse nature of the bands with polyatomic molecules are usually ambiguous, since it is not certain whether true cases of "predissociation" are involved or whether the large moments of inertia lead to unresolved close packing of rotational lines.

The absorption spectra of some polyatomic molecules containing methyl and ethyl radicals

Although measurements on the ultra-violet absorption spectra of polyatomic molecules have rapidly multiplied in recent years, probably in no case has the structure of the entire spectrum been satisfactorily and completely interpreted. From the chemical point of view, investi­gations have been mainly directed to the study of “predissociation” processes and their correlation with the primary processes of photo­ chemical change, whilst in addition some knowledge has been gained in regard to the products of photodissociation and energies of linkage. A more careful examination of the matter has now shown that the inferences to be drawn from predissociation phenomena must be made with care, and in many cases additional measurements—for example of fluorescence or of quantum efficiencies—have to be made before the interpretations become unambiguous. From the physical standpoint, only a few band systems have been analysed in detail ( e . g ., ClO 2 , Urey and Johnston*; SO 2 , Watson and Parker,) and even in these the interpretations given may not be accurate. One aspect of the matter which has not yet received much attention, is the nature and type of the vibrations excited in polyatomic molecules. This may prove to be of considerable importance in connection with chemical kinetics. The chief difficulty in the analysis of the spectra of polyatomic mole­cules usually arises from their complexity, whilst the frequent occurrence of purely continuous spectra which may or may not overlap band systems often makes it impossible to derive much knowledge of the molecular excited states. In such cases as the latter, it may be that further in the ultra-violet, i . e ., in the Schumann region, discrete band systems may lead to knowledge of higher electronic states, but this region has so far been little explored. Herzberg and Teller* have recently attempted to con­struct selection rules for electronic and vibrational transitions in poly­atomic molecules, but even when such rules as these are applied and when the infra-red and Raman frequencies are well known, the analysis of most band systems still remains very difficult.

Analysis of Complicated Band Spectra with the Aid of Magnetic Rotation Spectra

THE absorption spectra of polyatomic molecules are usually so complicated that an analysis of their rotational fine structure seems impossible. We have therefore tried if the magnetic rotation spectra which are obtained by putting the absorbing gas in a magnetic field between crossed nicols1 will not simplify those spectra sufficiently so that an analysis becomes possible. It was found that with NO2 a remarkable simplification occurred. But in this case also the simplified magnetic rotation spectrum is very complicated and we have not yet succeeded in analysing it, further experimental work under higher dispersion being required before an analysis can be attempted.

Absorption spectra of polyatomic molecules. Predissociation and dissociation of these molecules

Absorption spectra of polyatomic molecules. Predissociation and dissociation of these molecules