13 research outputs found
Six-fold-symmetry internal rotation in toluenes: the low barrier challenge of 2,6-and 3,5-difluorotoluene
Pure six-fold symmetry (V6) internal rotation poses significant challenges to experimental and theoretical determination, as the very low torsional barriers result in huge tunneling splittings difficult to identify and to model. Here we resolved the methyl group internal rotation dynamics of 2,6- and 3,5-difluorotoluene using a newly developed computer code especially adapted to V6 problems. The jet-cooled rotational spectra of the title molecules in the 5–25 GHz region revealed internal rotation tunneling doublings of up to 3.6 GHz, which translated in methyl group potential barriers of V6 = 0.14872(24) and 0.0856(10) kJ mol−1, respectively, in the vibrational ground-state. Additional information on Stark effects and carbon isotopic species in natural abundance provided structural data and the electric dipole moments for both molecules. Ab initio calculations at the MP2 level do not reproduce the tiny torsional barriers, calling for experiments on other systems and additional theoretical models.DFGMINECO/CTQ2012-39132-C02-0
Conformational steering in dicarboxy acids: the native structure of succinic acid
Succinic acid, a dicarboxylic acid molecule, has been investigated spectroscopically with computational support to elucidate the complex aspects of its conformational composition. Due to the torsional freedom of the carbon backbone and hydroxy groups, a large number of potentially plausible conformers can be generated with an indication that the gauche conformer is favored over the trans form. The microwave and millimeter wave spectra have been analyzed and accurate spectroscopic constants have been derived that correlate best with those of the lowest energy gauche conformer. For an unambiguous conformational identification measurements were extended to the monosubstituted isotopologues, precisely determining the structural properties. Besides bond distances and angles, particularly the dihedral angle has been determined to be 67.76(11)°, confirming the anomalous tendency of the methylene units to favor gauche conformers when a short aliphatic segment is placed between two carbonyl groups.Spanish Ministry of Science and Innovation/CTQ2011-22923Spanish Ministry of Science and Innovation/CGL2011-2244
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Photothermal Investigations on Certain Plasma Polymerized Thin Films and Near IR Overtone Studies of Some Organic Molecules
Photothermal effect refers to heating of a sample due to the absorption of
electromagnetic radiation. Photothermal (PT) heat generation which is an example of
energy conversion has in general three kinds of applications. 1. PT material probing
2. PT material processing and 3. PT material destruction. The temperatures involved
increases from 1-. 3. Of the above three, PT material probing is the most important
in making significant contribution to the field of science and technology.
Photothermal material characterization relies on high sensitivity detection techniques
to monitor the effects caused by PT material heating of a sample. Photothermal
method is a powerful high sensitivity non-contact tool used for non-destructive
thermal characterization of materials. The high sensitivity of the photothermal
methods has led to its application for analysis of low absorbance samples. Laser
calorimetry, photothermal radiometry, pyroelectric technique, photoacoustic
technique, photothermal beam deflection technique, etc. come under the broad class
ofphotothermal techniques. However the choice of a suitable technique depends upon
the nature of the sample, purpose of measurement, nature of light source used, etc.
The present investigations are done on polymer thin films employing photothermal
beam deflection technique, for the successful determination of their thermal
diffusivity. Here the sample is excited by a He-Ne laser (A = 6328...\ ) which acts as
the pump beam. Due to the refractive index gradient established in the sample surface
and in the adjacent coupling medium, another optical beam called probe beam (diode
laser, A= 6500A ) when passed through this region experiences a deflection and is
detected using a position sensitive detector and its output is fed to a lock-in amplifier
from which the amplitude and phase of the deflection can be directly obtained. The
amplitude and phase of the signal is suitably analysed for determining the thermal
diffusivity.The production of polymer thin film samples has gained considerable
attention for the past few years. Plasma polymerization is an inexpensive tool for
fabricating organic thin films. It refers to formation of polymeric materials under the
influence of plasma, which is generated by some kind of electric discharge. Here
plasma of the monomer vapour is generated by employing radio frequency (MHz)
techniques. Plasma polymerization technique results in homogeneous, highly
adhesive, thermally stable, pinhole free, dielectric, highly branched and cross-linked
polymer films. The possible linkage in the formation of the polymers is suggested by
comparing the FTIR spectra of the monomer and the polymer.Near IR overtone investigations on some organic molecules using local mode
model are also done. Higher vibrational overtones often provide spectral
simplification and greater resolution of peaks corresponding to nonequivalent X-H
bonds where X is typically C, N or O. Vibrational overtone spectroscopy of
molecules containing X-H oscillators is now a well established tool for molecular
investigations. Conformational and steric differences between bonds and structural
inequivalence ofCH bonds (methyl, aryl, acetylenic, etc.) are resolvable in the higher
overtone spectra. The local mode model in which the X-H oscillators are considered
to be loosely coupled anharmonic oscillators has been widely used for the
interpretation of overtone spectra. If we are exciting a single local oscillator from the
vibrational ground state to the vibrational state v, then the transition energy of the
local mode overtone is given by .:lE a......v = A v + B v2
• A plot of .:lE / v versus v will
yield A, the local mode frequency as the intercept and B, the local mode diagonal
anharmonicity as the slope. Here A - B gives the mechanical frequency XI of the
oscillator and B = X2 is the anharmonicity of the bond. The local mode parameters XI
and X2 vary for non-equivalent X-H bonds and are sensitive to the inter and intra
molecular environment of the X-H oscillator.Department of Physics,
Cochin University of Science and Technolog
Photothermal and photoacoustic investigations on certain polymers and semi conducting materials
Material synthesizing and characterization has been one of the major areas of
scientific research for the past few decades. Various techniques have been suggested for
the preparation and characterization of thin films and bulk samples according to the
industrial and scientific applications. Material characterization implies the determination
of the electrical, magnetic, optical or thermal properties of the material under study.
Though it is possible to study all these properties of a material, we concentrate on the
thermal and optical properties of certain polymers. The thermal properties are detennined
using photothermal beam deflection technique and the optical properties are obtained
from various spectroscopic analyses. In addition, thermal properties of a class of
semiconducting compounds, copper delafossites, arc determined by photoacoustic
technique.Photothermal technique is one of the most powerful tools for non-destructive
characterization of materials. This forms a broad class of technique, which includes laser
calorimetry, pyroelectric technique, photoacollstics, photothermal radiometric technique,
photothermal beam deflection technique etc. However, the choice of a suitable technique
depends upon the nature of sample and its environment, purpose of measurement, nature
of light source used etc. The polynler samples under the present investigation are
thermally thin and optically transparent at the excitation (pump beam) wavelength.
Photothermal beam deflection technique is advantageous in that it can be used for the
detennination of thermal diffusivity of samples irrespective of them being thermally thick
or thennally thin and optically opaque or optically transparent. Hence of all the abovementioned
techniques, photothemlal beam deflection technique is employed for the
successful determination of thermal diffusivity of these polymer samples. However, the semi conducting samples studied are themlally thick and optically opaque and therefore, a
much simpler photoacoustic technique is used for the thermal characterization.The production of polymer thin film samples has gained considerable attention
for the past few years. Different techniques like plasma polymerization, electron
bombardment, ultra violet irradiation and thermal evaporation can be used for the
preparation of polymer thin films from their respective monomers. Among these, plasma
polymerization or glow discharge polymerization has been widely lIsed for polymer thin
fi Im preparation. At the earlier stages of the discovery, the plasma polymerization
technique was not treated as a standard method for preparation of polymers. This method
gained importance only when they were used to make special coatings on metals and
began to be recognized as a technique for synthesizing polymers. Thc well-recognized
concept of conventional polymerization is based on molecular processcs by which thc
size of the molecule increases and rearrangemcnt of atoms within a molecule seldom
occurs. However, polymer formation in plasma is recognized as an atomic process in
contrast to the above molecular process. These films are pinhole free, highly branched
and cross linked, heat resistant, exceptionally dielectric etc. The optical properties like
the direct and indirect bandgaps, refractive indices etc of certain plasma polymerized thin
films prepared are determined from the UV -VIS-NIR absorption and transmission
spectra. The possible linkage in the formation of the polymers is suggested by comparing
the FTIR spectra of the monomer and the polymer. The thermal diffusivity has been
measured using the photothermal beam deflection technique as stated earlier. This
technique measures the refractive index gradient established in the sample surface and in
the adjacent coupling medium, by passing another optical beam (probe beam) through
this region and hence the name probe beam deflection. The deflection is detected using a
position sensitive detector and its output is fed to a lock-in-amplifIer from which the amplitude and phase of the deflection can be directly obtained. The amplitude and phase
of the deflection signal is suitably analyzed for determining the thermal diffusivity.Another class of compounds under the present investigation is copper
delafossites. These samples in the form of pellets are thermally thick and optically
opaque. Thermal diffusivity of such semiconductors is investigated using the
photoacoustic technique, which measures the pressure change using an elcctret
microphone. The output of the microphone is fed to a lock-in-amplificr to obtain the
amplitude and phase from which the thermal properties are obtained. The variation in
thermal diffusivity with composition is studied.Department of Physics,
Cochin University of Science and Technolog
Microwave rotational spectrum and ab initio computations on 4-cyanopyridine: molecular structure and hyperfine interactions
<p>The cm-wave rotational spectrum of 4-cyanopyridine was studied on a pulsed supersonic jet-expansion Fourier-transform microwave spectrometer. The ground state rotational parameters for the parent and four <sup>13</sup>C and two <sup>15</sup>N isotopologues were determined with high accuracy in natural abundance including all quartic centrifugal distortion constants and nuclear quadrupole coupling parameters of the two nitrogen nuclei. With the study of both singly substituted <sup>15</sup>N species, the nuclear origins of the hyperfine structures could be differentiated and the respective constants identified unambiguously. Different approaches have been used to determine the structure: empirical (<i>r</i><sub>s</sub>), ab initio and semi-experimental () methods. The semi-experimental approach improves the accuracy significantly, whereas the Kraitchman's method fails again.</p
MW SPECTROSCOPY COUPLED WITH ULTRAFAST UV LASER VAPORIZATION: SUCCINIC ACID IN THE GAS PHASE
Author Institution: Departamento de Quimica Fisica, Facultad de Ciencia y Tecnologia, Universidad del Pais Vasco (UPV/EHU), Ap.644, E-48940, Bilbao, Spain; School of Chemistry, Box 23 Victoria 3800 Monash University, Australia; Institut fur Physikalische Chemie und Elektrochemie, Gottfried-Wilhelm-Leibniz-Universitat, 30167 Hannover, GermanyRecent lab and field measurements have indicated critical roles of organic acids in enhancing new atmospheric aerosol formation. In order to understand the nucleation process, here we report an experimental and theoretical investigation of chemical structure of succinic acid. We have used the technique of Fourier Transform Microwave Spectroscopy (FTMW). Succinic acid was vaporized by UV ultrafast laser ablation to suppress thermal decomposition processes and seeded into an expanding stream of Ne forming a supersonic jet. The rotational spectrum detected the presence of a single most stable conformation in the cm- mm- wave regions for which accurate rotational and centrifugal distortion parameters have been determined. The study was extended to all monosubstituted isotopic species (, , ), which were positively identified, leading to an accurate determination of the effective and substitution structures of the molecule. The experimental study was supplemented by \emph{ab initio} (MP2) and DFT (M06-2X and B3LYP) calculations. E. J. Cocinero, A. Lesarri, P. Ecija, F. J. Basterretxea, J. U. Grabow, J. A. Fernandez and F. Casta\~{n}o, \emph{Angew. Chem. Int. Ed.}, 51, 3119-3124, 2012