1,721,117 research outputs found
L'effetto Raman di noncoincidenza della seconda armonica del modo vibrazionale di stretching C=O dell'acetone
No full textLa separazione spettrale tra i momenti primi della componente anisotropa ed isotropa della banda Raman associata ad un modo vibrazionale avente un elevato valore della derivata del momento di dipolo elettrico, cioè il cosiddetto effetto di noncoincidenza (NCE), è un’osservabile accessibile per via spettroscopica che rivela l’occorrenza di accoppiamenti vibrazionali di risonanti za tra molecole adiacenti all’interno di un liquido molecolare, vedi [1] e referenze ivi allegate.
In pubblicazioni antecedenti abbiamo riportato risultati sperimentali e di simulazione sull’effetto di noncoincidenza NCE(3) del modo vibrazionale di stretching 3 dell’acetone (C=O) in funzione della sua diluizione in miscele chimiche ed isotopiche [2-3].
Il presente lavoro si concentra sull’effetto di noncoincidenza NCE(23) della seconda armonica 23, il quale manifesta una separazione spettrale tra i momenti primi dei picchi anisotropi ed isotropi di entità comparabile a NCE(3), ma di segno opposto, causato dal picco isotropo 23 a circa 3413.9 cm-1 e dal picco anisotropo 23 a circa 3407.5 cm-1 [3].
Questo contributo scientifico presenta l’attuale stato dello studio sperimentale e di simulazione sul NCE(23).
Bibliografia
[1] Torii H., “Computational methods for analyzing the intermolecular resonant vibrational interactions in liquids and the noncoincidence effect of vibrational spectra”, in Novel Approaches to the Structure and Dynamics of Liquids: Experiments, Theories and Simulations, Samios J. and Durov V.A. (Eds.), Kluver, pp. 343-360 (2004).
[2] M. Musso, M.G. Giorgini, H. Torii, R. Dorka, D. Schiel, A. Asenbaum, D. Keutel, K.L. Oehme, J. Molec. Liquids 2006, 125, 115.
[3] M. Musso, M.G. Giorgini, H. Torii, J. Molec. Liquids 2009, 147, 37
L'organizzazione strutturale di liquidi dipolari in soluzioni elettrolitiche rivelata dall'effetto di noncoincidenza Raman: il caso ddelle soluzioni d Li+/Carbonato di Propilene (PC) e Li+/(PC+DMC)
No full textNegli ultimi anni sono state raccolte svariate evidenze che indicano che l’effetto di noncoincidenza (NCE), ovvero la separazione spettrale fra il profilo anisotropo ed isotropo di una banda Raman totalsimmetrica, costituisca un osservabile spettroscopico intimamente correlato con la struttura microscopica di un liquido molecolare. Nell’ambito di una estesa attività spettroscopica rivolta a stabilire questa correlazione, abbiamo recentemente indagato le alterazioni che NCE della banda (C=O) di solventi carbonilici subisce come conseguenza della presenza di ioni alcalini ed alcalino terrosi in soluzioni elettrolitiche [1,2]. L’uso congiunto di calcoli quanto chimici ci ha permesso di stabilire in modo inequivocabile una correlazione fra il valore negativo di NCE osservato per questa banda in una soluzione Li+/acetone e la struttura delle specie clusterizzate Mm+(acetone)n presenti nelle soluzioni elettrolitiche e di giustificare il progressivo aumento del valore negativo di NCE della banda (C=O) con l’aumento della densità di carica dello ione Mm+.
Per la rilevanza rivestita dal Carbonato di Propylene (PC) come solvente nello sviluppo di batterie secondarie per la sua elevata permettività elettrica (=64) abbiamo esteso lo studio di NCE a soluzioni elettrolitiche Li+/PC. Il valore di NCE che per la banda (C=O) di PC puro risulta piccolo e positivo (NCE=5 cm-1), e cambia in modo molto evidente in soluzione Li+/PC (xLi+=0.09), diventando negativo e molto grande (NCEexper. =-41 cm-1 ), come atteso, per la formazione di clusters Li+(PC)n. I calcoli quanto chimici dello spettro Raman anisotropo ed isotropo della banda (C=O) condotto per svariati clusters Li+/(PC)n (n=2, 3, e 4) indicano che l’osservazione di un valore negativo così elevato di NCE è compatibile solo con la formazione della specie con n=4 (NCEcalc(n=2)=....., NCEcalc (n=3), NCEcalc (n=4) = -35 cm-1) in cui i quattro gruppi C=O puntano tetraedricamente verso lo ione Li+ (gruppo di punti Td).
Abbiamo esteso questo tipo di indagine ad altri solventi carbolinilici come dietil (DEC) ed etil-metil (EC) carbonato che sono usati spesso come co-solventi nelle batterie al litio per ridurre la viscosità della soluzione elettrolitica Li/PC. Risultati preliminari condotti su una soluzione equimolare di PC/DEC sembrano indicare una solvatazione preferenziale di Li+ da parte del PC.
.
Bibliografia
[1] M. G. Giorgini, H. Torii, M. Musso, G. P. Venditti, J. Phys. Chem. B 112, 7506 (2008).
[2] M. G. Giorgini, H. Torii, M. Musso, Phys. Chem. Chem. Phys. 12, 183 (2010)
The structural organization of organic eklectrolytic solutions probed by the Raman noncoincidence effect: experimental and quantum chemical results
No full textThe structural organization of organic electrolytic solutions probed by the Raman noncoincidence effect: experimental and quantum chemical results.
M. G. Giorgini1, H. Torii2, M. Musso3
1 Dipartimento di Chimica Fisica ed Inorganica, Università di Bologna, Viale del
Risorgimento 4, I-40136 Bologna, Italy.
2 Department of Chemistry, School of Education, Shizuoka University, 836
Ohya, Shizuoka 422-8529, Japan.
3 Fachbereich Materialforschung und Physik, Abteilung Physik und Biophysik,
Universität Salzburg, Hellbrunnerstraße 34, A-5020 Salzburg, Austria
Electrolytic solutions (M+X/Solvent) are of noteworthy importance in both technological [1] and biological solvation processes [2]. The mobility of ions and their activity within a solution strongly depend on its structure at molecular level and then on the specific interactions between ions and between ions and solvent molecules. Ion pairs (M+--X) and solvent clusters M+(S)n formation is expected as a consequence of these interactions. Recently we have investigated the liquid structure of electrolytic solutions of mono-valent (M+ = Li+, Na+) salts in carbonyl solvents by making use of the Raman noncoincidence effect (NCE) of the solvent (C=O) band and, with the joint use of ab initio molecular orbital quantum-chemical calculations, we have been able to assess the formation of cluster species M+(S)n [3]. NCE, i.e. the difference between the spectral first moments, M, of the anisotropic and isotropic profiles of a Raman band of a totally symmetric vibrational mode, is indicated as a reliable probe of the liquid organization. We have found [3] that the observed large and negative NCE of the (C=O) band is a consequence of the formation of clusters species M+(S)n made of n C=O groups of solvent molecules (S) pointing towards the M+ cation in a tetrahedral (n = 4, Li+) and octahedral (n = 6, Na+) organization. Now we have extended these investigations to electrolytic solutions of bi-valent cations, Mg2+, Ca2+, Sr2+, and Ba2+, to shed light on the effect of the increased strength of the electric field on NCE and assess, on a quantum chemical basis, the formation of M2+(S)n clusters. Our observations indicate a remarkable increase of the negative NCE (34.2 cm-1 in Mg2+/acetone and 21.1 cm-1 in Ba2+/acetone solutions, in Figure 1a) as compared with those found in the singly charged ions (e.g., –16.0 cm-1 in Na+/acetone) [3].
The NCE calculated for the species (acetone)nMg2+ with n = 3, 4 and 6 suggests the formation, in the Mg2+/acetone solution, of the n = 6 cluster species (NCE= 35.1 cm-1) in which six carbonyl groups are pointing towards the Mg2+ ion in an octahedral organization, as illustrated in Figure 1b.
1. J. McBreen, H.S. Lee, X. Q. Yang, X. Sun, J. Power Sources 89, 163 (2000).
2. D. Vaden, J. M. Lisy, Chem. Phys. Lett. 408, 54 (2005).
3. M. G. Giorgini, H. Torii, G. Venditti, M. Musso, J. Phys. Chem B, 112, 7506 (2008).
Figure 1. a) Isotropic and anisotropic Raman profiles of the (C=O) band of acetone/M2+ solutions with M = Mg, Ca, Ba (in red, blue, and green, respectively), where B and C indicate the “bulk” and “cluster” components of the band, and those of neat liquid acetone (in black); b) Cluster of six acetone molecules pointing towards the Mg2+ ion in an octahedral organization
One- and two-mode behaviour of the (CO) Raman band in isotopic liquid mixtures of N,N-Dimethylformamide. Observation and computational prediction.
No full textAfter its observation in the phonon spectra of mixed crystals (alkali halides) and molecular crystals (metal-hexacarbonyls), the phenomenon of either one- or two-mode behavior of specific spectral bands has been observed in binary isotopic liquid mixtures. First noticed in the Raman spectrum of the amide I [(C=O)] band of formamide/formamide-d2 liquid mixtures [1], the one-mode behavior consists in the spectral merging of the bands associated with a given normal mode with slightly different intrinsic frequencies in the two neat liquids, to only one band with a frequency situated in between, in contrast to the expected observation of two separate (two-mode) bands attributable to the two species of the mixture. Useless to say that overlap of the two bands and dynamical D/H exchange (occurring in a timescale not accessible to Raman spectroscopy) can be excluded as possible origins of merged bands. Whether one or the other of the two behaviors is observed depends upon the occurrence of more or less strict resonance conditions, i.e., how close are the oscillator frequencies in the two neat liquids respect to the intermolecular vibrational coupling between them and the bandwidth.
To understand how the spectral features (resonance conditions and bandwidths) of the two oscillator in the two species of the binary mixtures control the degree of appearance of the one- and two-mode behavior, a computational procedure has been envisaged [2] based on the parametrization of the vibrational modes in a model liquid coupled by transition dipole mechanism (TDC). The application of this procedure to the isotropic and anisotropic spectra of the amide I band of N,N-dimethylformamide/ N,N-dimethylformamide-d1 and to N,N-dimethylformamide(12C=O)/ N,N-dimethylformamide (13C=O) mixtures (xmf=0, 0.25, 0.5, 0.75, 1.0) indicates the occurrence of the one-mode in the former and the two-mode behavior in the latter.
The results of preliminary Raman experiments carried out for the amide I band of the N,N-dimethylformamide/ N,N-dimethylformamide-d1 mixture at the same concentrations support the prediction of the one-mode behavior for this mode in this mixture. Experiments on the amide I band of the N,N-dimethylformamide(12C=O)/ N,N-dimethylformamide (13C=O) mixtures at the same concentrations are in progress.
REFERENCES
[1] Mortensen, A., Faurkov, O.F. , Yarwood, J., Shelley, V., ‘’Vibrational spectra of mixtures of isotopomers of formamide. Anomalies in the carbonyl stretching region ‘’, Journal of Physical Chemistry 98(20), 5221-5226(1994).
[2] Torii, H., Osada, Y., Iwami, M., ‘Merged and separate band profiles arising from resonantly coupled vibrational modes of liquid mixtures: theoretical study’, J. Raman Spectrosc. 39, 1592 (2008)
Microscopic inhomogeneities in liquid mixtures observed through the Raman spectroscopic noncoincidence effect
No full textThe Raman spectroscopic noncoincidence effect (NCE) describes the frequency separation between the first moments Maniso and Miso of the anisotropic and the isotropic components of the Raman band associated with the vibrational mode in a molecular liquid. The results reported are consistent with with the picture offered by the molecular dynamics simulation of the local solvent organization in acetone water solutions showing a clustering of the solution at all acetone concentrations
The effect of microscopic inhomogeneities in acetone/methanol binary mixtures observed through the Raman spectroscopic noncoincidence effect
No full textThe influence of microscopic inhomogeneities in the noncoincidence effect of the C=O stretch of acetone and on the O-H stretch of methanol in acetone/methanol mixtures has been studied as function of the concentration of the ywo liquids in the mixture. The observed convex concentration dependence of the noncoincidence effect of both vibtational bands is in agreement with that obtained for a Stockmayer mixture via Monte Carlo simulations,indicating the occurrence of microscopic inhomogeneities
Merged- and Separate-Band Behavior of the C=O Stretching Band in N,N-Dimethylformamide Isotopic Liquid Mixtures: DMF/DMF-d1, DMF/DMF-d6, and DMF/DMF-13C=O
No full textA combined experimental and theoretical analysis is carried out on the polarized (isotropic and anisotropic) Raman spectra in the spectral region of the C=O stretching (amide I) band of three isotopic liquid mixtures of N,N-dimethylformamide (normal/d1, normal/d6, and normal/13C=O). Two distinct types of spectral behavior are found for the isotropic Raman spectra: the separate-band behavior (for normal/13C=O), where two separate bands (one for each species) appear at all concentrations but with significant intensity bias, and the merged-band behavior (for normal/d6), where only one band appears at a frequency between those of the two species and with a band shape noticeably different from the simple overlap of their profiles. An intermediate case between these two limits is also found (for normal/d1). These main spectral features, as well as the noncoincidence effect (NCE) observed for all the mixtures and neat liquids, are well reproduced by the calculations, meaning that (1) the computational procedure (carried out in the time domain) incorporates all the important factors that determine the main spectral features, and (2) the band merger and the intensity bias are both controlled by the same type of term (resonant intermolecular vibrational coupling) of the vibrational Hamiltonian that gives rise to the NCE. Based on this result, the one- and two-dimensional infrared spectra of the normal/d1 1:1 mixture are calculated as theoretical predictions. For this purpose, an eigenstate-free method is developed to increase the efficiency of the time-domain spectral calculations and to do the calculations on a largest possible system. The calculated spectral features are compared with those of the polarized Raman spectra and discussed
Time-Domain Theoretical Analysis of the Noncoincidence Effect, Diagonal Frequency Shift, and the Extent of Delocalization of the C=O Stretching Mode of Acetone/Dimethyl Sulfoxide Binary Liquid Mixtures
No full textA time domain method for simulating vibrational band profiles that simultaneously takes into account both the diagonal and off-diagonal effects is developed and applied to the C=O stretching bands of neat liquid acetone and the acetone/dimethyl sulfoxide (DMSO) binary liquid mixtures. By using this method, it is possible to examine the influence of liquid dynamics on the noncoincidence effect (NCE), which arises from the off-diagonal vibrational interactions, as well as the frequency shifts and band broadening, which are related to both the diagonal and off-diagonal effects. It is shown that the simulations for the C=O stretching bands of acetone in acetone/DMSO binary liquid mixtures on the basis of this method can reproduce the experimentally observed concave curvature of the concentration dependence of the NCE and the unusually large frequency shift of the anisotropic Raman band. The widths of the infrared, isotropic Raman, and anisotropic Raman bands calculated for neat liquid acetone are also in good agreement with those observed. Based on these calculations, the extent of delocalization of the C=O stretching vibrational motions is examined by referring to two quantitative measures of this property, one calculated in the frequency domain and the other in the time domain. It is shown that the extent of delocalization gets larger as the mole fraction of acetone increases, the C=O stretching vibrations being delocalized over a few tens of molecules in neat liquid acetone. It is also shown that the extent of delocalization is related to the quantity called NCE detectability, which is the ratio between the magnitude of NCE and the band width. It is therefore suggested that the extent of delocalization of vibrational motions may be estimated from observable features of Raman band profiles
The influence of alkaline earth ions on the structural organization of acetone probed by the noncoincidence effect of the (C=O) band:experimental and quantum chemical results
No full textWe have investigated the Raman noncoincidence effect (NCE = ) of the (C=O) band arising from the interactions of acetone with the metal ions in acetone electrolytic solutions of alkaline earth metal (Mg, Ca, Sr, Ba) perchlorates. Assisted by the results of ab initio molecular orbital (MO) calculations carried out at the Hartree–Fock (HF) level with the 6-31+G(2df,p) and LanL2DZ basis sets, we have been able to attribute this band to the formation of acetone–metal ion clusters, (acetone)nM2+, and to interpret its high and negative NCE as the consequence of the large separation between the higher frequency of the in-phase mode (active in the Raman isotropic spectrum) and the lower (average) frequency of the n-1 out-of-phase modes (prevalently active in the Raman anisotropic spectrum). The negative sign of the NCE is compatible with the transition dipole coupling (TDC) mechanism. The comparison between the observed NCE for each electrolytic solution and those calculated for the different solvation numbers n of each (acetone)nM2+cluster gives a clear indication of the highest stability of the hexa-coordinated cluster for the Mg2+ ion, but leaving uncertain (n=6 or 8) this conclusion for the acetone clusters of the remaining M2+ ions. We have interpreted the observed and calculated decrease of the magnitude of NCE with the ion size through the ion polarizing power in the light of the ion effective charge and its distance (M2+--CO) from the CO oscillators
One- and two-mode behavior of the n(C=O) band in N,N-dimethylformamide isotopic mixtures: DMF/DMF-d1, DMF/DMF-d6, and DMF/DMF-13CO
No full text.Observation of the Raman spectrum of some binary isotopic liquid mixtures may cause a great surprise: the spectral merging of the bands associated with a given normal mode having slightly different intrinsic frequency in the two neat liquids, to only one band with a frequency located in between (one-mode behavior). It was noticed in the Raman spectrum of the amide I [(C=O)] band of formamide-d2/formamide-d3 liquid mixtures [1]. Both the overlap of the two bands and the dynamical D/H exchange could be excluded as possible origins of merging. The onset of this phenomenon requires the occurrence of very critical spectral conditions, because, while it is observed in the amide I band of the formamide-d2/formamide-d3 [1] mixtures, it is absent in the same mode of the analogous formamide/formamide-13CO mixtures [1] where the two-mode behavior is observed. To unveil the origin of this phenomenon and recognize which and how spectral conditions discriminate between the observation of the one- or two mode behavior, a computational procedure has been envisaged [2] based on the parametrization of the vibrational modes in a liquid coupled by transition dipole coupling mechanism (TDC). It is well known, in fact, that the (C=O) mode in liquid carbonyl compounds, and more notably in the amide I band present in amides, is affected by intermolecular resonant vibrational coupling as manifested by the frequency splitting between the Raman aniso- and isotropic components (termed noncoincidence effect, NCE), particularly large (15 cm-1) in these carbonyl compounds. It has been found [3] that the ingredient for the onset of the one-mode behavior is the extent of resonance condition (the magnitude of the resonant coupling compared with the separation of the wavenumber positions) between the vibrations of the two species and their bandwidths. With the present investigation we intend to analyse and discuss the one-/two-mode behaviour of the amide I band in isotopic mixtures of N,N-dimethylformamide (DMF) in the light of the general results obtained in [3] on the basis of a parametric analysis of a coupled oscillator system. Among the results obtained for the three types of isotopic mixtures under study we report, on the right, the case of the isotropic Raman spectra of DMF/DMF-d6 mixtures where the occurrence of the one-mode behavior is clearly seen. [1] Mortensen A., Faurskov Nielsen O., Yarwood J., Shelley V., J. Phys. Chem. 1995, 99, 4435-4440 [2] Torii H., J. Phys. Chem. A 2006, 110, 4822-4832. [3] Torii H., Osada Y., Iwami M., J. Raman Spectrosc. 2008, 39, 1592-159
- …
