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We employed vibrational sum-frequency generation (VSFG) spectroscopy to obtain molecular level understanding of interaction of l-phenylalanine (Phe) with lipid monolayers of zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at the air–water interface. The measured VSFG spectra in the CH stretch region due to the lipid and Phe, and the OH stretch region due to interfacial water molecules were analyzed. These results in combination with surface pressure studies reveal that the Phe molecules at acidic pH of 5.6 intercalate into DPPC monolayers, and replace some interfacial water molecules. Consequently, there is a decrease in the VSFG intensity in the OH stretch region in the Phe subphase, and a concomitant increase in the surface pressure of the DPPC monolayer. The exclusion of the water molecules is controlled by both the bulk concentration of Phe, and the surface concentration of DPPC. In contrast, at the neutral pH of 7.3 there is an increase in the VSFG intensity due to the interfacial wat...

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.

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.

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.

Butadiene monoxide (BMO) undergoes the S(0)--S(1) transition, involving the excitation of both pi and n electrons to pi(*) orbital, at 193 nm. After relaxing to the ground electronic state via internal conversion, BMO molecules undergo intramolecular rearrangement and subsequently dissociate to form unexpected OH radicals, which were detected state selectively by laser-induced fluorescence technique, and the energy state distribution was measured. OH is produced vibrationally cold, OH(nu(")=0,J(")), with the rotational population characterized by a rotational temperature of 456+/-70 K. The major portion (approximately 60%) of the available energy is partitioned into internal degrees of the photofragments, namely, vibration and rotation. A considerable portion (25%-35%) also goes to the relative translation of the products. The Lambda doublet and spin-orbit ratios of OH were measured to be nearly unity, implying statistical distribution of these states and, hence, no preference for any of the Lambda doublet (Lambda+ and Lambda-) and spin-orbit (Pi(3/2) and Pi(1/2)) states. Formation time of the nascent OH radical was measured to be100 ns. Different products, such as crotonaldehyde and methyl vinyl ketone, were detected by gas chromatography as stable products of photodissociation. A reaction mechanism for the formation of all these photoproducts, transient and stable, is proposed. The multiple pathways by which these products can be formed have been theoretically optimized, and energies have been calculated. Absorption cross section of BMO at 193 nm was measured, and quantum yield of OH generation channel was also determined.