Variable Temperature Raman Study Research Articles

Exceptionally high temperature spin crossover in amide-functionalised 2,6-bis(pyrazol-1-yl)pyridine iron(ii) complex revealed by variable temperature Raman spectroscopy and single crystal X-ray diffraction.

The synthesis of a novel amide-functionalised 2,6-bis(pyrazol-1-yl)pyridine-4-carboxamide ligand (bppCONH2) is described. The complex salts [Fe(bppCONH2)2](BF4)2 and [Fe(bppCONH2)2](ClO4)2 were synthesised and characterised by SQUID magnetometry, differential scanning calorimetry, variable temperature Raman spectroscopy and single crystal X-ray diffraction. DSC measurements of [Fe(bppCONH2)2](BF4)2 indicate a spin-crossover (SCO) transition with T↑ at 481 K and T↓ at 461 K, showing a 20 K hysteresis. DSC for the perchlorate salt shows an SCO transition with T↑ at 459 K and T↓ at 445 K with a 14 K hysteresis. For the BF4- salt analysis of low and high-spin state crystal structures at 101, 290 and 500 K, suggest stabilisation of the low spin state due to the formation of 1D hydrogen-bonded cationic chains. Variable temperature Raman studies of the BF4 salt support the presence of a high temperature SCO. It is speculated that the presence of hysteresis may be attributed to differences in the inter-molecular hydrogen bonding in the low spin and high spin states.

A Known Iron(II) Complex in Different Nanosized Particles: Variable-Temperature Raman Study of Its Spin-Crossover Behavior.

The spin-crossover (SCO) polymorph B (complex 1) of the known compound [FeII{N(CN)2}2(abpt)2], where abpt is 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, has been prepared in three different particle sizes averaging ∼300 (sample 1a), ∼80 (sample 1b), and ∼20 nm (sample 1c). Two independent octahedral molecules possessing Fe1 and Fe2 were found to be present in the crystal of B. Magnetostructural relationships had established that at room temperature both FeII sites are in the high-spin state (HS-HS), whereas a decrease in the temperature to 90 K induces the complete high-spin to low-spin conversion of the Fe1 site, with Fe2 remaining in the high-spin state (LS-HS). The three samples have been characterized by elemental analyses, ATR spectra, solution UV/vis spectra (to exclude resonance Raman effects) and powder X-ray diffraction patterns, while their morphological characteristics have been examined by scanning electron microscopy (SEM). The SCO behavior of the originally prepared sample 1a has been monitored in detail by variable-temperature Raman studies in the 300-80 K range using mainly low-frequency ν(Fe-N) and δ(NFeN) modes and the ν(C≡N) mode of the axial dicyanamido groups as spin-sensitive vibrations. The new peaks that appear in the low-temperature Raman spectra of the LS-HS form of the complex are reproduced in the calculated spectrum of the LS state of [FeII{N(CN)2}2(abpt)2]. The influence of the average particle size on the SCO properties of 1 has also been studied by variable-temperature Raman spectra. The studies indicate that, during the HS-HS → LS-HS transition, the latter form of the complex appears at higher temperatures for the smaller particles; the T1/2 shift accomplished by manipulating the particle size within a range of roughly 1 order of magnitude (300-20 nm) may be as high as ∼30 K. The SCO features of 1, as deduced from the Raman study, are in excellent agreement with those derived from a traditional variable-temperature magnetic susceptibility study, indicating the utility of the former.

Investigating the Recrystallization Behavior of Amorphous Paracetamol by Variable Temperature Raman Studies and Surface Raman Mapping

In situ Raman spectroscopy and Raman mapping are used to monitor the crystallization of amorphous paracetamol in both covered and uncovered geometries, for which different crystallization pathways have been reported previously. The results suggest that surface crystallization predominates in the uncovered samples, leading to forms I and II, whereas in the covered samples bulk crystallization dominates and leads to form III.

Structural Studies on the Basic Ionic Liquid 1-Ethyl-1,4-diazabicyclo[2.2.2]octanium Bis(trifluoromethylsulfonyl)imide and Its Bromide Precursor

The structure of 1-ethyl-1,4-diazabicyclo[2.2.2]octanium bis(trifluoromethylsulfonyl)imide ([C2DABCO][NTf2]) was determined by single crystal X-ray diffraction, and its behavior with temperature was examined by synchrotron powder diffraction, differential scanning calorimetry (DSC), and Raman spectroscopy. [C2DABCO][NTf2] undergoes a solid–solid phase transition at 309 K and a solid–liquid (melting) transition at 350 K, as reported previously in the literature. The crystal structure of [C2DABCO][NTf2] was determined at temperatures below the solid–solid transition (solid-I phase), whereas in situ synchrotron powder diffraction enabled the unit cell of the solid phase observed above the solid–solid transition temperature (solid-II phase) to be indexed. High-resolution synchrotron powder diffraction data indicated the coexistence of a metastable phase with the solid-I and solid-II phases around the temperature of the solid–solid phase transition. A variable temperature Raman study suggested rotational movem...

ChemInform Abstract: Photolysis of Noble Gas-Fluorine Mixtures; Dimorphism in Krypton Fluoride Chemistry.

The UV photolysis of krypton–fluorine mixtures at –196°C readily affords KrF2. Variable-temperature Raman studies on this material indicate a temperature-dependent phase transition at –80°C. The low-temperature UV photolysis of krypton–fluorine mixtures in the presence of Lewis acids (AsF5 or SbF5) only affords KrF2. Variable-temperature Raman studies on the resulting mixtures show that Kr2F3+SbF6–, α- and β-KrF+Sb2F11–, KrF+AsF6– and KrF+As2F11– are formed on warming the mixtures.

Raman spectroscopic studies on structure I and structure IItrimethylene oxide hydrate

Raman spectra were collected from structure I (sI) and structure II (sII) trimethylene oxide (TMO) hydrates at various temperatures and used to assign the vibrational modes, investigate the ordering of TMO molecules inside the cage structure, and to determine possible interactions between the guest and the host lattice. Only Raman spectra from sI hydrate could be analyzed since the low concentration of TMO prevented sII peaks from being resolved. Comparison of the Raman spectra of liquid, solid, and enclathrated TMO (sI) showed Raman shifts to higher wave numbers for the enclathrated TMO; mainly among the ring modes. The ring and (or) skeletal modes around 930 cm–1 and the asymmetric CH2 stretching mode around 2970 cm–1, showed shifts greater than 10 cm–1. These observed changes are interpreted on the basis of the "loose-cage – tight-cage" model, which interprets the shift as arising from strain induced by the hydrate cage on the guest TMO molecules in the 62 cages. In addition, variable temperature Raman studies, in the temperatures ranging from 103 to 203 K, showed no evidence of ordering and (or) reorientation of host molecules. PACS No.: 78.30.-j

Molecular structure of 1,1,2,2-tetra-tert-butyldisilane: unusual structural motifs in sterically crowded disilanes.

The molecular structure of 1,1,2,2-tetra-tert-butyldisilane has been determined by gas-phase electron diffraction supported by ab initio calculations, in the solution phase by Raman spectroscopy, and in the solid phase by Raman spectroscopy and X-ray crystallography. The gas-phase structure (C2 symmetry) was found to be almost anticlinal, a most unusual and unexpected result. In the favoured conformation, contact between tert-butyl groups at each end of the molecule is avoided by a large deviation of the angles around the silicon atoms from the parent tetrahedral angle of 109.5 degrees. In fact, the Si-Si-C angles returned from the gas electron diffraction refinement are 117.0(5) and 110.7(6) degrees, indicating the large degree of flexibility about the silicon centres. The ab initio methods and gas electron diffraction results indicate that there is only one conformer of But2HSiSiHBut2 in the gaseous mixture. Variable temperature Raman studies indicate the possibility of a further higher energy conformer existing in the liquid phase. However, this seems quite improbable from other observations made for the Raman spectra at all temperatures. The X-ray structure is close to that observed in the gas phase, with phiHSiSiH = 94.2(18) degrees. There is a large amount of disorder about one of the silicon postions and one of the tert-butyl groups within the crystal structure, which makes detailed direct comparison with the gaseous structure difficult.

Perchlorovinylsilane (Cl2CCCl−SiCl3): Conformational Structure, Vibrational Analysis, and Torsional Potential Determined by Gas-Phase Electron Diffraction, ab Initio Calculations, and Variable Temperature Raman Spectroscopy

The molecular structure, conformation, vibrational spectra, and torsional potential of perchlorovinylsilane (PCV), Cl2CCCl−SiCl3, have been studied by using gas-phase electron diffraction (GED) data at 100 °C and variable temperature Raman spectroscopy, together with ab initio molecular orbital calculations. The GED data were treated by using a dynamic theoretical model. This involves fitting a chosen two-term potential function to the experimental data, thereby obtaining values for both a 3-fold and a 6-fold potential constant (V3 and V6) in the series V(φ) = 1/2ΣiVi[1 − cos i(180−φ)], where φ is the value of the torsional angle CCSiCl. According to the GED refinements, this molecule exists in the gas phase at 100 °C as a mixture of two minimum-energy conformers, syn (torsional angle φ(CCSiCl) = 0° or 120°) and anti (torsional angle φ(CCSiCl) = 180°), where the anti form predominates, occupying approximately 80% of the gas composition. Relevant structural parameters are as follows (anti): Bond lengths (rg): r(Si−C) = 1.863(13) Å, r〈(Si−Cl)〉 = 2.020(3) Å (average value), r(CC) = 1.349(12) Å. Bond angles (∠α): ∠〈CSiCl〉 = 111.1(15)°, ∠CCSi = 124.0(12)°. Error limits are given as 2σ (σ includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data). The estimated experimental conformational energy difference obtained from GED is ΔE°A-S = −1.04(±0.58) kcal/mol, based on the refined value of the V3 potential constant. From the variable temperature Raman study, two corresponding energy differences obtained from two separate pairs of doublets in the liquid phase are ΔE°A-S = −0.30(1) and ΔE°A-S = −0.43(5) kcal/mol. The ab initio value (HF/6-31G(d)) was ΔE°A-S = −1.43 kcal/mol. All results suggest that anti is the low-energy form. Full geometry optimizations were performed for seven pseudoconformers (including 120° and 180° forms), which were employed in the dynamic GED model, by using the ab initio MO HF/6-31G(d) level of theory. Scaled HF zero-point vibrational energy corrections were estimated from frequency calculations. The theoretical results are compared with experimental observations.

Spectra and structure of organophosphorus compounds. LIII. Infrared and Raman spectra, conformational stability, vibrational assignment and ab initio calculations of n-propyldichlorophosphine

The Raman (3100 to 50 cm −1) spectra of the liquid and solid and the infrared (3100 to 50 cm −1) spectra of the gaseous and solid n-propyldichlorophosphine, C 3H 7PCl 2, have been recorded. Additionally, the infrared (3100 to 400 cm −1) spectrum of the sample dissolved in liquid xenon has been obtained. Qualitative Raman depolarization ratios have been reported for the liquid phase. The spectra of the fluid phases have been interpreted on the basis of three out of five possible conformations. In the most stable trans-trans (TT) conformer, the phosphorus electron lone pair is in a trans position relative to the ethylene group (first trans) and the conformation of the heavy atom skeleton is also trans (second trans). The next most stable conformer is the gauche-trans (GT) rotamer where the enthalpy difference has been determined to be 1.68 ± 12 cm −1 (481 ± 35 cal mol −1) from a variable-temperature Raman study in the liquid phase and 105 ± 26 cm −1 (302 ± 75 cal mol −1) from a variable-temperature study of the infrared spectra of the sample dissolved in liquid xenon. Enthalpy differences have also been obtained for the gauche-I-gauche (G 1G) conformer which is third strongest in stability. In the solid phase, only the TT conformer has been identified. A complete vibrational assignment is proposed for the TT conformer as well as for some of the fundamentals of the other two more stable rotamers. The structural parameters, rotational constants, dipole moments and energies have been obtained from ab initio calculations employing the RHF/3-21G∗, RHF/6-31G∗ and MP2/6-31G∗ basis sets. The vibrational assignments are consistent with the calculated (RHF/3-21G∗ with two scaling factors) fundamental vibrational frequencies. These results are compared with similar quantities obtained for n-propylphosphine and ethyldichlorophosphine.

Conformational analysis, ab initio calculations, and vibrational assignment of 3-chloropropionitrile

Abstract Raman (3200 to 10 cm −1 ) and infrared (3200 to 400 cm −1 ) spectra have been recorded for liquid and solid 3-chloropropionitrile, ClCH 2 CH 2 CN. These data are interpreted on the basis that the molecule exists as a mixture of trans (Cl atom oriented trans to the CN group) and gauche conformers in the liquid phase with the gauche form the more stable rotamer and the only conformer in the solid. From variable temperature Raman studies of the liquid, the enthalpy difference between conformers is determined to be 609 ± 31 cm −1 (1.74 ± 0.09 kcal mol −1 ). A complete vibrational assignment is proposed for both conformers. We have carried out ab initio calculations employing the MP2/6-31G* basis set to obtain the structural parameters, barriers to internal rotation, the fundamental frequencies for both conformers, and the infrared and Raman intensities. The calculated values are compared with the corresponding experimental quantities. These results are discussed and compared with similar quantities obtained for some related molecules.

Spectra and structure of organophosphorus compounds. LI. IR and Raman spectra, conformational stability, barriers to internal rotation, vibrational assignment, and ab initio calculations of n-propylphosphine

Abstract The Raman (3100 to 50 cm−1) and IR (3100 to 50 cm−1) spectra of gaseous and solid n-propylphosphine, C3H7PH2, and the corresponding P-d2 isotopomer have been recorded. Additionally, the Raman spectra of the liquids have been obtained with qualitative depolarization ratios. From these data, all five possible conformers have been identified in the fluid states and the trans-trans conformer is shown to be the most stable rotamer in both the gaseous and liquid states and it is the only conformer present in the solid. The first trans refers to the orientation of the lone pair to the ethylene group (rotation around the PC bond) whereas the second trans refers to the orientation of the methyl group relative to the PC bond (rotation around the −CH2CH2 bond). The next most stable conformer is the gauche-trans rotamer where the enthalpy difference has been determined from variable-temperature Raman studies to be 140 ± 5 cm−1 (400 ± 14 cal mol−1) for the vapor and 351 ± 20 cm−1 (1004 ± 57 cal mol−1) for the liquid. The other three conformers have nearly the same stabilities but significantly higher energies than the two more stable rotamers. From the far-IR data and relative conformer stabilities, some of the coefficients of the potential function governing conformer interconversion are estimated. A complete vibrational assignment is proposed for the trans-trans conformer and for the fundamentals for most of the heavy atom motions for the other conformers. The conformational stabilities, barriers to internal rotation, and fundamental vibrational frequencies which have been determined experimentally are compared to those obtained from ab initio calculations employing the RHF/3-21G* and/or RHF/6-31G* basis sets. Additionally, the conformational stabilities and structural parameters have been determined with the 6-31G* basis set with electron correlation at the MP2 level. These results are compared with the corresponding quantities for some similar molecules.

Infrared and Raman spectra, conformational stability, barriers to internal rotation, ab initio calculations and vibrational assignment of ethyl methyl selenide

The Raman spectra (3200–10 cm −1) of ethyl methyl selenide in the gas, liquid and solid phases and the infrared spectra (3200–30 cm −1) of the gas and solid have been recorded. Qualitative depolarization ratios have been obtained for the lines in the Raman spectrum of the liquid. By a variable temperature Raman study of the liquid, it has been determined that the gauche conformer is more stable than the trans rotamer by 158±16 cm −1 (452±46 cal mol −1), and the gauche conformer is the rotamer present in the solid. A complete vibrational assignment for the gauche conformer is presented. All of these data are compared to the corresponding quantities obtained from ab initio Hartree—Fock gradient calculations employing the STO-3G* and 4–31G*/MIDI-4* basis sets. Complete equilibrium geometries have been calculated for both rotamers and the results are discussed and compared with the corresponding quantities for some similar molecules.

Photolysis of noble gas–fluorine mixtures; dimorphism in krypton fluoride chemistry

The UV photolysis of krypton–fluorine mixtures at –196°C readily affords KrF2. Variable-temperature Raman studies on this material indicate a temperature-dependent phase transition at –80°C. The low-temperature UV photolysis of krypton–fluorine mixtures in the presence of Lewis acids (AsF5 or SbF5) only affords KrF2. Variable-temperature Raman studies on the resulting mixtures show that Kr2F3+SbF6–, α- and β-KrF+Sb2F11–, KrF+AsF6– and KrF+As2F11– are formed on warming the mixtures.

Raman spectra and ab initio calculations for methyl nitrite and methyl‐d3 nitrite

AbstractThe Raman (4000–10 cm−1) and infrared (4000–80 cm−1) spectra of methyl nitrite, CH3ONO, and methyl nitrite‐d3 were recorded in the gas and solid phases. Additionally, the Raman spectra of the pure liquids were recorded and qualitative depolarization values were obtained. Support for the interpretation of these new data is provided by ab initio calculations using the 6–31G* basis set. Complete equilibrium geometries have been determined for both conformers, as well as structures for every 30° rotation about the NO bond. These calculations give a cis/trans barrier of 4015 cm−1 (11.5 kcal mol−1), a trans/cis barrier of 3504 cm−1 (10.0 kcal mol−1) and an enthalpy difference between conformers of 511 cm−1 (1.5 kcal mol−1) with the cis form being more stable. From a variable temperature Raman study of liquid methyl nitrite‐d3 it is concluded that the cis form is also more stable in this phase with an enthalpy difference between conformers of 262 ± 3 cm−1 (750 ± 9 cal mol−1). A complete assignment of the vibrational fundamentals is proposed based on band contours, depolarization values, group frequencies and shift factors for the two isotopomers. These assignments are supported by normal coordinate analyses using the ab initio force fields to obtain frequencies and potential energy distributions for both conformers.

Far-infrared spectra and barriers to internal rotation of ethyl nitrate

The far-infrared spectra of gaseous and solid ethyl nitrate, CH 3CH 2ONO 2, have been recorded from 500 to 50 cm −1. The fundamental asymmetric torsion of the trans conformer which has a heavy atom plane has been observed at 112.50 cm −1 with two excited states failing to lower frequencies, and the corresponding fundamental torsion of the gauche conformer was observed at 109.62 cm −1 with two excited states also falling to lower frequencies. The results of a variable temperature Raman study indicate that the trans conformer is more stable than the gauche conformer by 328 ± 96 cm −1 (938 ± 275 cal mol −1). An asymmetric potential function governing the internal rotation about the CH 2O bond is reported which gives a trans to gauche barrier of 894 ± 15 cm −1 (2.56 ± 0.04 kcal mol −1) and a gauche to gauche barrier of 3063 ± 68 cm −1 (8.76 ± 0.20 kcal mol −1) with the trans conformer more stable by 220 ± 148 cm −1 (0.63 ± 0.42 kcal mol −1). Transitions arising from the symmetric CH 3 and NO 2 torsions are observed for both conformers, from which the threefold and twofold periodic barriers to internal rotation have been calculated. For the trans conformer the values are 1002 cm −1 (2.87 kcal mol −1) and 2355 ± 145 cm −1 (6.73 ± 0.42 kcal mol −1) and for the gauche conformer they are 981 cm −1 (2.81 kcal mol −1) and 2736 ± 632 cm −1 (7.82 ± 1.81 kcal mol −1) for the CH 3 and NO 2 rotors, respectively. These results are compared to the corresponding quantities for some similar molecules.

Infrared and raman spectra, conformational stability and normal coordinate analysis of ethyldimethylamine-borane

The IR (3200-80 cm −1) spectra have been recorded for gaseous and solid ethyldimethylamine-borane, CH 3CH 2(CH 3) 2N(BH 3), and its deuterated analogue, CH 3CH 2(CH 3) 2N(BD 3). Additionally, the Raman (3200-10 cm −1) spectra have been recorded for the liquid and solid phases, with qualitative depolarization values being obtained from the liquid. Based on the disappearance of several bands on going from the gas or liquid to the solid, it is concluded that the molecule exists as a mixture of the trans and gauche conformers in the fluid phases, but only the gauche conformer is present in the solid. From a variable temperature Raman study of the liquid the ethalpy difference is estimated to be 113±150 cm −1 (323±429 cal mol −1). A complete vibrational assignment is proposed for the gauche conformer based on group frequencies, isotopic shift data, and relative intensities. The assignment is supported by a normal coordinate calculation which was carried out utilizing a modified valence force field to calculate the frequencies of the normal modes and the potential energy distribution. The asymmetric torsion has been assigned to the bands at 130 and 124 cm −1 in the d 0 and d 3 compounds, respectively. These results are compared with corresponding results for several other amine-boranes and phosphine-boranes.

Far infrared and low frequency Raman spectra and conformational stability of gaseous chloromethylcyclopropane and bromomethylcyclopropane

The far-IR (350-50 cm −1) and low frequency Raman spectra (200-70 cm −1) of gaseous chloro- and bromomethylcyclopropane have been recorded. The asymmetric torsion arising from the gauche conformer of the bromide has been observed at 91.8 cm −1 but the corresponding transition for the cis conformer was not observed. However, IR bands for all the skeletal bending modes indicate the presence of a second conformer at ambient temperature. For chloromethylcyclopropane, the asymmetric torsions of both the cis and the gauche conformers have been observed at 99.54 and 96.84 cm −1, respectively, along with two excited state transitions for each conformer. From these data a potential function for the chloromethylcyclopropane molecule has been calculated and the determined potential constants have the following values: V 1=200±65, V 2=−34±16, and V 3=1138±6 cm −1 with the gauche conformer calculated to be more stable by 178±87 cm −1 (510±25 cal mol −1) than the cis conformer. The gauche to cis barrier is 1239 cm −1 (3.54 kcal mol −1) and the gauche to gauche barrier is 1114 cm −1 (3.19 kcal mol −1). A variable temperature Raman study of the gaseous chloride was carried out and the enthalpy difference was found to be Δ H=273±108 cm −1 (782±309 cal mol −1) with the gauche conformer being more stable. The results of this study are compared with previous studies of these and similar molecules.

A variable-temperature Raman study of vibrational modes in NH4I, suggesting the existence of a new low-temperature phase

There are four well studied solid phases of NH4I in the temperature and pressure range 150 to 300 K and 0 to 10 kbar. Several years ago a fifth phase was reported by Heyns and co-workers (1980) of pressure higher than 30 kbar. The authors present evidence that suggests another phase transition at about 110 K and 0 kbar.

Spectra and structure of phosphorus—boron compounds Part XXV. Infrared and Raman spectra, conformational stability and normal coordinate analysis of ethyldimethylphosphine—borane

The Raman (10 to 3500 cm −1) and infrared (50 to 3500 cm −1) spectra of solid ethyldimethylphosphine—borane, CH 3CH 2(CH 3) 2P(BH 3) and its deuterated analog, CH 3CH 2(CH 3) 2P(BD 3) have been recorded. Additionally, the Raman spectra of the liquid phases have been recorded and qualitative depolarization ratios have been obtained. Based on the fact that several bands were observed to disappear upon going from the liquid to the solid phase, it is concluded that the molecule exists as a mixture of the trans and gauche conformers with the trans conformer being the only one present in the solid phase. Employing Raman depolarization data, group frequencies and isotopic shift data, the spectrum of the solid phase has been interpreted on the basis of C s symmetry for the trans conformer. The assignment is supported by a normal coordinate calculation which was carried out utilizing a modified valence force field to obtain the frequencies of the normal modes and the potential energy distribution. The asymmetric torsion has been assigned to the bands at 127 cm −1 in both the -d 0 and -d 3 compounds. From a variable temperature Raman study it is concluded that the gauche conformer is the more stable form in the liquid phase and the enthalpy difference between the two conformers is estimated to be 590 ± 100 cm −1 (1.7 ± 0.3 cal mol −1). These results are compared with corresponding results for several other organophosphine—borane compounds.

Variable temperature Raman study of urea

Detailed Raman spectra of oriented single crystals of urea-H 4 and urea D- 4 have been recorded at various temperatures between 77–350K. These spectra together with their corresponding polarized Raman spectra at 293K and 77K have been used in the assignments of the low frequency spectrum of urea.