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[3+2] Cycloadditions of nitrones as three-atom components to alkenes yield isoxazolidine cycloadducts, which on chemical transformations can be converted to bioactive compounds. The [3+2] cycloadditions route thus provides conversion of simple natural products to more complex naturally occurring bioactive nitrogen heterocycles, and close analogues. As α,β-unsaturated amides abundantly occur as natural products, [3+2] cycloadditions of nitrones with simpler α,β-unsaturated amides were studied to get information about reactivity profiles. The reactions of C-(4-chlorophenyl)-N-methyl nitrone as three-atom component to cinnamic acid anilides were investigated. The 3,4-trans-4,5-trans-4- carboxanilido-2-methyl-3,5-diaryl isoxazolidines were the major cycloadducts; the diastereoisomeric 3,4-cis-4,5-trans-4-carboxanilido-2-methyl-3,5-diaryl isoxazolidines and regioisomeric 3,4-trans-4,5- trans-5-carboxanilido-2-methyl-3,4-diaryl isoxazolidines were obtained as minor cycloadducts. The cycloadducts were characterized by NMR studies and XRD analysis.

Investigations of [3+2] cycloadditions of C,N-diaryl and C-aryl-N-methyl nitrones as three atom components (TAC) to substituted methyl E-cinnamates and diethyl arylidene malonates have been further investigated. [3+2] Cycloadditions of cinnamates yielded mixtures of cycloadducts, the major products being the 3,4-trans-4,5-trans-2,3,5-triaryl-4-carbomethoxy products originating from the endo-carbonyl-exo-aryl meta channel approach of the cinnamate component. [3+2] Cycloadditions to diethyl arylidene malonates furnished single cycloadducts-3,5-trans-2-methyl-3,5-diaryl-4,4- dicarbethoxy isoxazolidines by a endo-aryl meta channel approach of the 2π-component.

Trifluoroacetic acid (TFA) is released in the atmosphere through its use in the chemical industry and as degradation product of chlorofluorocarbon (CFC) alternatives like hydrofluorocarbons and hydrochlorofluorocarbons. In the present study, we have investigated the OH formation dynamics in the photodissociation of TFA at 193 nm by Laser Photolysis-Laser Induced Fluorescence (LP-LIF) method, as well as stable product formation by GC MS and FTIR. It was found that, ∼26% of the available energy is partitioned into the relative translation of the photoproducts (f(T) = 0.26), which could be explained by presence of an exit barrier of ∼13 kcal/mol in OH formation channel. This result is very similar to OH formation from acetic acid (AA) and difluoroacetic acid (DFA), indicating fluorination at the side chain of aliphatic carboxylic acids does not significantly change the mechanism of C OH bond scission. Our experimental results tallied with the theoretical studies, which suggested that the major OH formation channel in acetic acid and fluoroacetic acid is direct dissociation from the optically excited S1 state through an exit barrier, with some competition from the T1 state. However, quantum yield of OH formation from TFA (0.4) was found to be much smaller than AA (0.8), which is probably caused by higher reaction barrier in T1 state of TFA, compared to AA. CHF3, C2F4, C2F6, CO2, CO, CF3CFO, CF2O and hexafluoropropylene oxide (HFPO) were detected as the stable products of the photolysis of TFA. The theoretically optimized ground state dissociation channels showed significant difference between TFA and AA.

The rate coefficients for reactions of 1-chlorocyclopentene with OH have been measured in the temperature range of 262–335 K, at total pressure of 30 Torr, using Laser Photolysis-Laser Induced Fluorescence. The rate coefficient can be expressed as k(T)=(6.32 ± 1.16) × 10-12 exp((669 ± 45)/T) cm3molecules-1s−1, and thus it depends weakly on temperature. It decreases with increasing temperature, which was attributed to formation of a pre-reactive complex between reactants. Molecular orbital calculations confirm participation of such a complex during an addition reaction, and also predict several other hydrogen abstraction pathways. The studies suggest the dominance of addition reaction over H atom abstraction in the reaction of 1-chlorocyclopentene with OH.

We employed vibrational sum-frequency generation (VSFG) spectroscopy to obtain molecular-level understanding of interaction of anionic surfactant sodium dodecyl sulfate (SDS), in low bulk concentration at the micromolar level, with lipid monolayer zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at the air–water interface. These results are different from those reported at higher bulk concentration of SDS at millimolar level. At very low concentration neither DPPC nor SDS produces any VSFG signal in the CH stretch region in the water subphase, but with the same concentration DPPC produces typical VSFG spectra at the SDS subphase due to interaction between these two molecules. The interaction leads to polar ordering of DPPC molecules with enhancement of VSFG intensity in the CH vibrational region of the hydrophobic tails. The interaction between the lipid and SDS molecules is influenced by concentrations of both lipid and SDS. Hydrophobic interactions between long alkyl chains of SDS and DPP...

The structure and orientation of interfacial morpholine molecules have been investigated, using vibrational sum-frequency generation (VSFG) spectroscopy—a nonlinear surface specific technique. The VSFG spectra with SSP and PPP polarizations have been measured in the CH (2800–3000 cm–1) and the OH (3000–3750 cm–1) stretch regions at the air–morpholine and air–solution interfaces. The vibrational frequencies in the CH stretch region could be observed in VSFG spectra, implying presence of morpholine molecules at the interfaces with net polar orientation. The intensities of the CH stretch bands get enhanced at the air–solution interface of morpholine solution in millimolar concentration, in comparison to that at the air–morpholine interface which is attributed to increase in polar orientation of interfacial morpholine molecules induced by water molecules. In pure morpholine, the most predominant conformation of molecules is equatorial chair, both in the bulk and at the air–morpholine interface. But in aqueous...

The structure and orientation of room temperature ionic liquid (RTIL) 1‐butyl‐3‐methylimidazolium tris(pentafluoroethyl)trifluorophosphate [PF 3 (C 2 F 5 ) 3 ], commonly known as [bmim][fap], have been investigated at the air−[bmim][fap] and air–water interfaces, employing vibrational sum–frequency generation (VSFG) spectroscopy. The VSFG spectra in the CH stretch region suggest presence of the [bmim] cation at the interfaces. Studies reveal that the butyl chain protrudes out into air, and the imidazolium ring lies almost planar to the interface. The CH stretch intensities get enhanced at the air–water interface, mainly because of polar orientation of imidazolium cation induced by interfacial water molecules. The OH stretch intensities are also enhanced at the air–water interface due to polar orientation of interfacial water molecules induced by [bmim][fap]. The Brewster angle microscopy suggests self aggregation of [bmim][fap] in the presence of water, and the aggregation becomes extensive showing dense surface domains with time. However, the surface pressure is almost unaffected due to aggregation.

The rate coefficient for the gas phase reaction of the OH radical with 1,2-propylene oxide (PPO) has been measured at 298 K using a relative rate method, employing Gas Chromatography (GC) technique, using ethane and 1,2-dichloroethane as reference compounds. Temperature dependence of the above reaction is also studied using an absolute rate method, employing Laser Photolysis-Laser Induced Fluorescence (LP-LIF) technique, over a range of 261–335 K. The measured bimolecular rate coefficients were fitted into the Arrhenius plot, which yielded a straight line roughly parallel to the x-axis and implies no or very weak temperature dependence. Within the experimental error limits, rate coefficients obtained at various temperatures, are found to vary within a narrow range from (4.65 ± 0.24) × 10−13 to (4.80 ± 0.24) × 10−13 cm3 molecule−1 s−1. Room temperature rate coefficient of the reaction of PPO with the OH radical was found to be (4.65 ± 0.24) × 10−13 cm3 molecule−1 s−1, using LP-LIF technique. These results have been discussed with available literature to unveil the probable mechanism. An average tropospheric lifetime of PPO has been estimated to be about two weeks for degradation by the OH radical. In addition, atmospheric parameters such as Radiative Efficiency (RE) and Global Warming Potential (GWP) for PPO are determined. Stable products formed are characterized using GC-MS and FT-IR techniques. Theoretical calculations are performed to understand the energetics and mechanism of H atom abstraction by OH, and also to complement the experimental observations.

Morpholine is a very interesting cyclic compound which is an ether as well as a secondary amine. The relative distribution of different conformations of morpholine depends on the medium. Using Raman spectroscopy and theoretical calculations, we found that, equatorial chair conformer is predominant in the pure liquid, but in aqueous solution, contribution from the axial conformer increases. The surface-enhanced Raman scattering (SERS) studies in presence of silver nanoparticles showed change in relative intensities of different vibrational modes of morpholine. It was found that the axial chair conformer is preferentially adsorbed vertically through the N Ag bond on the nanoparticle surface.

Dynamics of OH radical formation in the 193 nm photolysis of 2-methoxyethanol is studied using Laser Photolysis–Laser Induced Fluorescence technique. The nascent state distribution of the OH radical is measured. The OH fragments are formed vibrationally cold, characterized by a Boltzmann-like single rotational temperature of 450 ± 100 K. The spin–orbit and Λ -doublet ratios of OH fragments are measured. The relative average translational energy of the OH channel is determined to be 17.0 ± 3.0 kcal/mol. The experimental studies along with theoretical calculations suggest a complex mechanism for OH formation consisting of at least three pathways. The prominent pathway at shorter timescale ( 1 ms) involves a series of reaction with initial H3C–OCH2CH2OH bond cleavage, followed by rearrangement of OCH2CH2OH to CH2OCH2OH, and a final concerted step to generate OH and ethylene epoxide.

Piperazine exists in a number of energetically close structural conformations, and here, we investigated the dependence of their relative abundance on the surrounding conditions by using Raman and SERS spectroscopy in pure solid, aqueous solution and Ag hydrosol. The experimental results were interpreted by DFT calculations using B3LYP functional with aug-cc-pvdz/LANL2DZ basis sets. In the chair form of piperazine, which is more stable than the skewed boat by ∼8 kcal mol −1 , the two N–H bonds can remain equatorial or axial, leading to three different conformations, eq–eq, eq–ax and ax–ax. The calculated Raman spectrum of the lowest energy eq–eq conformation corresponds well with the experimental spectrum in pure solid, indicating eq–eq to be predominant. But, the contribution of the eq–ax conformation was found to be maximum in aqueous solution. The SERS spectrum revealed that eq–ax conformation was preferably adopted as piperazine was adsorbed vertically through its axial N-atom over silver nanoparticle surface.

The photodissociation dynamics of benzoyl chloride at 235 nm has been investigated and compared with that of 2-furoyl chloride. Atomic Cl and molecular HCl channels have been detected in benzoyl chloride by employing resonance-enhanced multiphoton ionization technique and time-of-flight mass spectrometry. Both the Cl fragments, Cl((2)PJ=3/2, relative quantum yield 0.70 ± 0.15) and Cl*((2)PJ=1/2), show isotropic angular distribution and bimodal translational energy distributions. The predominant high kinetic energy channel contributes 72% to the C-Cl bond scission and arises from the S1 state having nπ* character of benzoyl chloride. However, the low-energy Cl and HCl channels originate from the ground electronic state. The most plausible mechanism of HCl formation is proposed based on molecular orbital calculations. In contrast to benzoyl chloride, the HCl channel is not observed in 2-furoyl chloride on excitation at 235 nm, and this is attributed to an energy constraint.

The laser photolysis-laser-induced fluorescence method was used for measuring the kinetic parameters of the reaction of OH radicals with CF3CH2OCH2CF3 (2,2,2-trifluoroethyl ether), in the temperature range of 298–365 K. The bimolecular rate coefficient at 298 K, kII(298), was measured to be (1.47 ± 0.03) × 10−13 cm3 molecule−1 s−1, and the temperature dependence of kII was determined to be (4.5 ± 0.8) × 10−12exp [−(1030 ± 60)/T] cm3 molecule−1 s−1. The error quoted is 1σ of the linear regression of the respective plots. The rate coefficient at room temperature is very close to the average of the three previous measurements, whereas the values of Ea/R and the A-factor are higher than the two previously reported values. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 519–525, 2010

Photodissociation dynamics of morpholine at 193 nm was studied, using laser induced fluorescence technique. OH photoproduct was detected, though the parent molecule does not contain any OH group. The nascent OH radical was found to be in electronically and vibrationally ground state. Experimental results indicate that the major part, i.e., ∼35% of the available energy is partitioned as the relative translational energy of the photofragments. Between the two spin–orbit states, 2 Π 3/2 and 2 Π 1/2 , the latter is preferentially populated. The two Λ-doublet states are, however, found to be statistically populated. Based on both the theoretical and the experimental studies, a suitable reaction mechanism has been proposed for OH formation.

Photodissociation of styrene oxide at 193 nm in gas phase generates OH, as detected by laser-induced fluorescence technique. Under similar conditions, OH was not observed from ethylene and propylene oxides, primarily because of their low absorption cross-sections at 193 nm. Mechanism of OH formation involves first opening of the three-membered ring from the ground electronic state via cleavage of either of two C O bonds, followed by isomerization to enolic forms of phenylacetaldehyde and acetophenone, and finally scission of the C OH bond of enols. Ab initio molecular orbital calculations support the proposed mechanism.

Kinetics of reaction of OH radical with morpholine, a heterocyclic molecule with both oxygen and nitrogen atoms, has been investigated using laser photolysis-laser-induced fluorescence technique, in the temperature range of 298-363 K. The rate constant at room temperature (k(298)) is (8.0 +/- 0.1) x 10(-11) molecule(-1) cm(3) s(-1). The rate constant decreases with temperature in the range studied, with the approximate dependence given by k(T) = (1.1 +/- 0.1) x 10(-11) exp[(590 +/- 20)/T] cm(3) molecule(-1) s(-1). The rate constants are high compared with those of similar heterocyclic molecules with oxygen atom but comparable to those reported for aliphatic amines. Ab initio molecular orbital calculations show that prereactive complexes, 5-7 kcal mol(-1) lower in energy as compared with the reactants, are formed because of hydrogen bond interaction between OH and the N/O atom of morpholine. The stability of the complex involving the nitrogen atom is found to be more than that involving the oxygen atom. The optimized transition-state structures and energies for the different pathways of hydrogen abstraction from these prereactive complexes explain the observation of negative activation energy.

Dynamics of formation of electronically excited NO2 and formation of OH fragment, during photo dissociation of 2-nitropropane (NP) and 2-methyl-2-nitropropane (MNP), were investigated at 193 and 248 nm. The radiative lifetime of the electronically excited NO2 fragment, observed at 193 nm, was measured to be 1.2 ( 0.1 micros and the rate coefficient of quenching of its emission by MNP was measured as (2.7 ( 0.1) x 10(-10) molecule(-1) cm3 s(-1). Formation of the ground electronic state of OH was confirmed in both molecules. State selective laser induced fluorescence technique was used to detect the nascent OH (X 2Pi, v'', J'') fragments in different ro-vibrational states, and to obtain information on energy partitioning. Though MNP and NP differ in the types of the available H atoms, the dynamics of OH formation is found to be the same in both. The relative population in different rotational states does not follow Boltzmann equilibrium distribution in both the molecules at 193 and 248 nm. The translational energies of the OH fragments, calculated from the Doppler width, are 21.2 ( 7.2 and 25.0 ( 2.5 kcal mol-1 for NP at 248 and 193 nm, respectively. The translational energies of the OH fragments, in the case of MNP, are found to be lower, 17.5 ( 4.1 and 22.0 ( 3.2 kcal mol-1,respectively, at 248 nm 193 nm. These results are compared with the earlier reports on photodissociation of nitromethane (NM), nitroethane (NE), and other nitroalkanes. All possible dissociation pathways of these molecules--NM, NE, NP, and MNPs leading to the formation of the OH fragment were investigated computationally, with geometry optimization at the B3LYP/6-311+G(d,p) level and energy calculation at the MP4(SDQ)/6-311+G (d,p) level. The results suggest that in NM, OH is formed after isomerization to CH2N(OH)O, whereas in all other cases OH is formed from HONO, a primary product of molecular elimination of nitroalkanes, formed with sufficient internal energy.

Tetrahydropyran (THP) undergoes photodissociation on excitation with ArF laser at 193 nm, generating OH radical as one of the transient photoproducts. Laser-induced fluorescence technique is used to detect the nascent OH radical and measure its energy state distribution. The OH radical is formed mostly in the ground vibrational level (v"=0), with low rotational excitation. The rotational distribution of OH (v"=0,J) is characterized by a temperature of 433+/-31 K, corresponding to a rotational energy of 0.86+/-0.06 kcalmol. Two Lambda-doublet levels, 2Pi+(A') and 2Pi-(A"), and the two spin-orbit states, the 2Pi(3/2) and 2Pi(1/2), of OH are populated statistically for all rotational levels. The relative translational energy associated with the photoproducts in the OH channel is calculated to be 21.9+/-3.2 kcal mol(-1), from the Doppler-broadened linewidth, giving an ft value of approximately 43%, and most of the remaining 57% of the available energy is distributed in the internal modes of the other photofragment, C5H9. The observed distribution of the available energy is explained well, using a hybrid model of energy partitioning, with an exit barrier of 40 kcal mol(-1). The potential-energy surface of the reaction channel was mapped by ab initio molecular-orbital calculations. Based on experimental and theoretical results, a mechanism for OH formation is proposed. Electronically excited THP relaxes to the ground electronic state, and from there, a sequence of reactions takes place, generating OH. The proposed mechanism first involves C-O bond scission, followed by a 1,3 H atom migration to O atom, and finally, the C-OH bond cleavage giving OH.

On excitation at 193 nm, tetrahydrofuran (THF) generates OH as one of the photodissociation products. The nascent energy state distribution of the OH radical was measured employing laser induced fluorescence technique. It is observed that the OH radical is formed mostly in the ground vibrational level, with low rotational excitation (approximately 3%). The rotational distribution of OH (v"=0,J) is characterized by rotational temperature of 1250+/-140 K. Two spin-orbit states, 2Pi3/2 and 2Pi1/2 of OH are populated statistically. But, there is a preferential population in Lambda doublet levels. For all rotational numbers, the 2Pi+(A') levels are preferred to the 2Pi-(A") levels. The relative translational energy associated with the photoproducts in the OH channel is calculated to be 17.4+/-2.2 kcal mol-1, giving an fT value of approximately 36%, and the remaining 61% of the available energy is distributed in the internal modes of the other photofragment, i.e., C4H7. The observed distribution of the available energy agrees well with a hybrid model of energy partitioning, predicting an exit barrier of approximately 16 kcal mol-1. Based on both ab initio molecular orbital calculations and experimental results, a plausible mechanism for OH formation is proposed. The mechanism involves three steps, the C-O bond cleavage of the ring, H atom migration to the O atom, and the C-OH bond scission, in sequence, to generate OH from the ground electronic state of THF. Besides this high energy reaction channel, other photodissociation channels of THF have been identified by detecting the stable products, using Fourier transform infrared and gas chromatography.