Vibrational Sum-Frequency Generation Study of Morpholine at Air

Article pubs.acs.org/JPCC

Vibrational Sum-Frequency Generation Study of Morpholine at Air− Liquid and Air−Solution Interfaces Sumana SenGupta, Ankur Saha, Awadhesh Kumar,* and P. D. Naik Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India S Supporting Information *

ABSTRACT: 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 solution of morpholine, the contribution from axial chair conformer is known to increase. This effect, and also a probable change from the chair to the boat/twist boat conformation at the air−solution interface, may contribute to the enhanced intensity of VSFG peaks of CH stretch bands in solution. The VSFG intensities of the OH stretching frequencies of interfacial water molecules are also enhanced in the presence of morpholine, suggesting an increase in net polar orientation of water molecules induced by morpholine molecules. The VSFG spectra were also measured in the presence of 300 mM HCl, which showed indications of protonation of the interfacial morpholine molecules. Addition of HCl to aqueous solution of morpholine alters the orientation of interfacial water molecules significantly, and the enhanced VSFG intensities in the OH region induced by morpholine molecules are almost completely obliterated. The result suggests that orientation of interfacial water molecules in the presence of HCl gets random. However, the effects of HCl in the CH region of the VSFG spectra differ for different stretching bands. In presence of readily ionizable HCl molecules, a large number of ions are generated, which are probably responsible for changing the surface orientation of both water and morpholine molecules.

1. INTRODUCTION Though an interfacial region between bulk phases consists of only a small fraction of the bulk material, they are very important because many biological and environmental reactions predominantly occur at the interfaces. Still, probing of the structure and properties of molecules on the interfaces remained a challenge for very long time because the small signal from the interfacial region gets completely masked by interferences from the bulk media for conventional spectroscopic techniques like IR and Raman. However, in the past three decades, the techniques based on second-order nonlinear optical processes, such as second harmonic generation (SHG) and sum-frequency generation (SFG) have been developed as very effective surface selective techniques for probing chemical bonding, structure, and molecular interaction at the interfaces. These processes are forbidden under the electric−dipole approximation in the bulk of media having inversion symmetry. At the interface between the centrosymmetric media, the symmetry is broken, permitting SHG and SFG processes. Thus, © 2016 American Chemical Society

they can specifically probe any interface accessible by light without any interference from the bulk phases.1,2 These techniques are interface specific, sensitive, and selective and, hence, ideal to study the microscopic structure and morphological properties of both insoluble (Langmuir) and adsorbed (Gibbs) monolayers over a broad range of fundamentally and technologically important surfaces and interfaces. We have, in this study, used the SFG technique to study the structure and behavior of morpholine molecules on air−liquid and air−morpholine solution interfaces under different conditions. Morpholine and its derivatives have various practical applications3,4 in corrosion protection, organic syntheses as catalysts and precursors, drugs and pharmaceuticals. Morpholine is often used in conjunction with low concentrations of Received: May 27, 2016 Revised: August 19, 2016 Published: August 24, 2016 20132

DOI: 10.1021/acs.jpcc.6b05351 J. Phys. Chem. C 2016, 120, 20132−20141

Article

The Journal of Physical Chemistry C

which is the sum of the visible (ωVIS) and IR (ωIR) input frequencies. The reflected IR and visible beams were blocked, and the SFG signal beam was separated and detected with a photomultiplier tube (PMT) after spatial, polarization and spectral filtering. Four different polarization schemes (SSP, SPS, PSS, and PPP) are possible for SFG experiments, and these are denoted based on the polarization states of the SFG, visible, and IR beams in the sequence. For example, the polarization SSP implies that the SFG and visible beams are s-polarized, whereas the IR beam is p-polarized. We have used both SSP and PPP polarization combinations in our experiments. For recording the vibrational spectra at an interface, the IR wavelength was scanned in the appropriate range, and the spectral resolution is determined by the line width of the IR laser. The CH and OH vibrational stretching regions were scanned in the range of 2750−3000 cm−1 and 3000−3750 cm−1, respectively. Each scan was obtained with a step size of 2 or 4 cm−1 for the CH and OH spectral regions, respectively, and an average of 60 laser shots per point in each spectrum. Morpholine sample (Sigma-Aldrich, 99% purity) was used without further purification, and stored in a desiccator when not in use. For VSFG studies at the air−morpholine interface, 10 mL of the neat liquid morpholine was taken in a circular glass trough (50 mm in diameter, and 27 mL volume) by a micropipet. For studies at the air−solution interface, required volume of the pure morpholine sample was measured out by suitable micropipets and added to 10 mL of Millipore water taken in the same glass trough. Calculated volume of the acid was also added to 10 mL of morpholine solution taken in the glass trough by the same method. All solutions were equilibrated for ∼12 h before recording spectra. All the experiments were conducted in the circular glass trough, which was mounted on a six-axis mount for spatial overlapping of the laser beams at the interface. We have also measured FTIR spectra of morpholine in the range of 650 to 4000 cm−1 in attenuated total reflectance (ATR) mode at room temperature using a FTIR spectrometer (Bruker, IFS 66v/S), equipped with a DTGS (deuterated triglycine sulfate) detector. In addition, we recorded the surface pressure of aqueous solutions of morpholine at different concentrations at room temperature (∼298 K), employing a platinum Wilhelmy plate microbalance with an accuracy of ±0.02 mN/m.

hydrazine or ammonia to provide comprehensive all-volatile treatment chemistry for corrosion protection for the steam systems of both fossil fuel power plant and nuclear power plants. Morpholine decomposes reasonably slowly in the absence of O2 at the high temperature and pressure in these steam systems.5 In addition to its practical applications, morpholine is a very interesting molecule from the viewpoint of fundamental chemistry because it is a cyclic compound with an ether linkage as well as a secondary amine, which can have a number of conformations in the ground state. The different conformations have difference in their physical properties like dipole moment and have different relative distribution depending on the nature of the medium. Thus, structure and orientation of morpholine molecules at different surfaces and interfaces are very important in understanding and optimizing their various applications. Raman spectra of morpholine molecules adsorbed on noble metal nanoparticles have been reported to be different for different concentrations of aqueous morpholine solution,6 and explained based on existence of different conformations and orientations of adsorbed molecules.6,15 Since nature of the surface plays an important role in determining the structure and orientation of an adsorbed molecule, the vibrational spectra of morpholine molecules at the air−solution interface can be different from that at the air− morpholine interface or in bulk solution. We have measured vibrational spectra of interfacial morpholine molecules at the air−morpholine and air−solution interfaces, employing vibrational sum-frequency generation (VSFG) spectroscopy. We have measured VSFG spectra of its dilute aqueous solution (10−60 mM concentration) in the CH (2800−3000 cm−1) and the OH (3000−3750 cm−1) stretch regions, and also investigated the influence of HCl solution on the spectral features.

2. EXPERIMENTAL SECTION: VIBRATIONAL SUM-FREQUENCY GENERATION SET-UP Vibrational sum-frequency generation (VSFG) is a nonlinear optical technique that selectively probes the vibrational spectra of molecules present exclusively at the surface.2,7 This is primarily a three-wave mixing process with two input beams and the third signal beam. The two input beams, IR and visible, interact temporally and spatially in a medium at the interface, and the coherent SFG signal is generated due to nonzero second-order nonlinear susceptibility, χ(2), at the interface. The experimental setup used in this work has already been described in detail in a previous report.8 Briefly, the visible beam with 30 ps temporal width is fixed at 532 nm, and it is generated by frequency doubling of the fundamental output of a Nd:YAG laser (PL2241B, Ekspla, Lithuania). The other input beam is tunable in IR frequency region of 2.3−10.0 μm spectral range. The tunable IR beam is generated in a difference frequency generator (DFG) by mixing the output of an optical parametric generator (OPG) with the fundamental output (1064 nm) of the Nd:YAG laser in a silver thiogallate (AgGaS2) crystal. The OPG (PG401, Ekspla) was pumped by the third harmonic (355 nm) beam of the Nd:YAG laser, using lithium triborate (LiB3O5) as a nonlinear crystal. These two input beams are passed through apertures, energy attenuators and polarizers, and finally loosely focused at the interface. The angles of incidence were kept at 55° and 60° for the IR and visible laser beams, respectively. The visible beam is also passed through a delay line for proper temporal overlap of both the beams at the interface. The SFG signal is generated at the frequency, ωSFG,

3. RESULTS AND DISCUSSION Morpholine is a six-membered heterocyclic molecule like cyclohexane with an O atom and an N−H bond replacing a pair of opposite −CH2 groups (Figure 1). It can exist in chair or skew boat conformers. It is a well established fact that the boat conformations are higher in energy compared to the chair conformations,6 and hence, they have no contribution in the IR or microwave spectra of morpholine.3,9 Therefore, this conformation has been considered negligible for bulk studies until now in literature. There can be multiple chair conformations, due to different orientations of the N−H bond with respect to the plane of the ring. The most important conformers are the ones with the N−H bond in the axial or the equatorial position. These two conformers, as shown in Figure 1, are called conventionally axial chair conformation or equatorial chair conformation. Energy difference between these two chair conformers is very low (around 1 kcal/mol in gas-phase as calculated at MP2/aug-cc-pvdz level6), the 20133


Article


as Raman spectra in liquid and crystalline phases.11 They have assigned the peaks in these spectra to the vibrational peaks of the axial and equatorial chair conformers. Their assignment of Raman spectra is in agreement with that of Xie et al.6 The region of 2600−3000 cm−1 is found to contain different stretching vibrations of CH2 groups of both the conformers. The N−H bond vibrations of the two conformers are seen as two distinct peaks above 3200 cm−1. 3.1. ATR-FTIR Spectra of Morpholine. FTIR spectra of morpholine in the range of 650 to 4000 cm−1 were measured with a resolution of 2 cm−1 in ATR mode at room temperature. The ATR cell was made of a trapezoidal ZnSe crystal with an incident angle of 45°. The vibrational frequencies of the CH stretch (2600−3000 cm−1) of pure liquid morpholine, its aqueous solution and solution in aqueous HCl are listed in Table 1. All the vibrational frequencies of liquid morpholine is shifted to higher wavenumber (blue-shifted up to 30 cm−1) in aqueous solution, implying weaker interaction between morpholine and water molecules than between morpholine molecules themselves. The vibrational frequencies of morpholine in aqueous solution are almost similar to that in HCl solution, except for the two lower frequencies at 2692 and 2759 cm−1, which are red- and blue-shifted by 10 and 5 cm−1, respectively. Similarly, on comparison of ATR-FTIR spectra of liquid morpholine with IR spectra,11 it is observed that most of the frequencies remain almost unchanged. But the ATR-FTIR frequency observed at 2676 cm−1 is red-shifted by 10 cm−1 from corresponding IR peak, whereas the ones at 2743 and 2829 cm−1 are blue-shifted by about 6 cm−1. These spectral shifts suggest that the molecular structure of morpholine in the bulk liquid and at the air−morpholine interface is different. 3.2. VSFG Spectra of Pure Liquid Morpholine. 3.2.A. Region between 2700 and 3000 cm−1. We recorded the VSFG spectra of pure liquid morpholine at room temperature in both SSP and PPP polarizations. The region between 2700 and 3000 cm−1 (Figure 2) exhibits generally the C−H stretching frequencies of the methyl (−CH3), methylene (−CH2), and methyne groups (−CH). Since morpholine

Figure 1. Different conformers of morpholine optimized at MP2/augcc-pvdz level of theory: (A) equatorial chair form; (B) axial chair form; (C) twist boat form: The dotted line is a pictorial representation of air−solution interface with air above and morpholine solution below the line.

equatorial one being more stable due to less steric interaction. In the Raman and IR spectra of pure liquid morpholine, contribution from both of these conformations can be detected.6,10,11 Vedal et al. reported the IR spectra of morpholine in gaseous, liquid and crystalline phases as well

Table 1. Observed C−H Stretch Vibrational Frequencies of Pure Liquid Morpholine, Its Aqueous Solution, and Aqueous Solution in the Presence of HCl in ATR-FTIR and VSFG Spectra observed C−H stretch vibrational frequencies (cm‑1) of morpholine VSFGb

ATR-FTIR liquida low fit − 2684 2746 2830 2851 2904

high fit 2631 2658 2676 2743 2746 2829 2852 2893 2910

(2628) (2661) (2686) (2737) (2748) (2822) (2851) (2893) (2911)

solution

HCl(aq.)

air−solution

air−HCl(aq.)

− 2692

− 2682

air−morpholine − −

− −

− −

2759

2764

2791 (2793)

−

−

2860 2868 2926

2860 2867 2924

2826 (2826) 2855 (2857) 2872 (2876)

2966

2963

2916 (2918) 2947

2945 (2943) (2951)

2942 (2946) 2969 (2969)

− 2854 2883 − 2926 2945 2969

(2825) (2855) (2882) (2912) (2923) (2945) (2971)

− 2851 2880 2894 2932 2953 −

(2832) (2851) (2877) (2894) (2933) (2956) (2971)

First and second columns represent frequencies obtained on low- and high-resolved Gaussian fit, respectively, to experimental spectra. Frequencies in parentheses are IR data from the literature.11 bVibrational frequencies are with the SSP (PPP) polarization.

a

20134


Article


−CH2 stretching modes are expected to have higher intensity in SSP polarization than PPP, while opposite is observed for the peak at 2791 cm−1. Since the existing studies on the correlation between the relative intensity of VSFG peaks at different polarizations and the symmetry of the vibrational modes are mostly based on straight chain compounds, more theoretical and experimental information is needed for unequivocally assigning the symmetry properties of the VSFG peaks of cyclic molecules, like morpholine. 3.2.B. Region between 3000 and 3900 cm−1. The N−H vibrations6,11 of the morpholine molecules typically occur in wavenumber range of 3290−3340 cm−1. In IR and Raman spectra, the N−H stretching peak of the equatorial chair conformer of morpholine is about 30 cm−1 blue-shifted compared to the axial conformer, thus their relative intensity in different polarizations is expected to give important information on the orientation and relative abundance of the conformers on the air−liquid interface. In our VSFG measurements in this region, negligibly small signal is observed in the SSP polarization (Figure 3a). However, a weak and broad feature in the region of 3000−3850 cm−1 is observed in the PPP polarization (Figure 3b). The broad features of the PPP spectra for the N−H bond stretch may arise out of extended hydrogen bonding between the surface molecules, and/or from broad orientational distributions of interfacial molecules. Because of such broad nature of the VSFG peak in N−H stretching region, the different conformers of morpholine molecules on the interface cannot be distinguished, unlike in case of Raman spectra. 3.3. VSFG Spectra of Morpholine Aqueous Solution. Morpholine is readily soluble in water, and forms hydrogen bonds through both its N−H center and O-center in presence of highly polar H2O molecules. In the Raman studies, it is observed that there is a change in the relative populations of the two chair conformers in solution as compared to liquid morpholine.6 The same study also reported that, in an aqueous solution, all morpholine peaks due to −CH2 stretching undergo blue-shift indicating the interaction between individual morpholine molecules is stronger than that between morpholine molecules and H2O molecules. In order to understand whether these bulk phase observations also work for molecules on the interface, we studied the VSFG spectra of very dilute solution of morpholine, in the concentration range 10−60 mM in both SSP and PPP polarization combinations. 3.3.A. Region between 2700 and 3000 cm−1. We collected the VSFG spectra of very dilute aqueous solution of morpholine, in the range 10−60 mM at an interval of 10 mM in both SSP and PPP polarization combinations. Typical VSFG spectra of the CH2 stretch region in both the polarizations for air−solution interface are compared with that of the air−pure morpholine interface in Figure 2, and individual spectra for all the concentrations in SSP polarizations are depicted in Figure 4. From these two figures, three significant features can be observed. From Figure 2, it can be observed that the VSFG intensities of morpholine solutions are greater than that of neat morpholine, and the difference between the spectra in SSP and PPP polarizations are higher with respect to that in neat liquid morpholine. From Figure 4, we can see that, in general, VSFG intensities of the CH2 stretch frequencies increase with increasing bulk concentration of morpholine solution. The increase in SFG signal intensity with increasing concentration can be attributed to the increase in the surface concentration of morpholine molecules as the bulk

Figure 2. CH stretching frequencies in VSFG spectra at air− morpholine and air−solution interfaces with SSP and PPP polarizations. Solid and dotted lines are fit to the experimental data.

molecule has only CH2 groups, all the peaks in this region must belong to different CH2 stretching vibrations. The IR and Raman peaks of pure morpholine in vapor, liquid, and crystalline forms are assigned exhaustively by Vedal et al.11 In the IR and Raman spectra of the pure liquid morpholine, the contribution from both equatorial and axial chair conformation was detected.6,11 Because of such complexity as well as comparatively poorer characterization of vibrational modes of a ring compound compared to that of open chain molecules, individually assigning the VSFG peaks to particular vibrational modes in the molecule, becomes complicated. In this case, it is more useful and sensible to assign the spectral features to their individual symmetries as demonstrated by Lu et al.12,13 and Buchbinder et al.14 Lu et al. have proposed some polarization selection rules for VSFG vibrational peaks which are valid for copropagating geometry of the two incident lasers (visible and IR). They have studied the polarization dependent VSFG spectra of aliphatic alcohols on liquid vapor surface and observed that the intensity of different peaks vary with different polarizations. The difference in the intensity between SSP and PPP spectra was in orders of magnitude.12,13 In case of pure morpholine, as can be seen from Figure 2, the difference in the intensity with SSP and PPP polarizations is not so well-defined for all peaks. From Figure 2, we can see seven vibrational resonances in the SSP(PPP) spectrum at 2791 (2793), 2826 (2826), 2855 (2857), 2872 (2876), 2916 (2918), 2942 (2946), and 2969 (2969) cm−1, and these are listed in Table 1. These resonances in two polarizations have different relative amplitudes, but similar frequencies, as expected. On the basis of the polarization selection rules and nature of these bands in Raman spectra, these stretching modes can be assigned either symmetric or asymmetric vibrational modes. In the Raman spectrum of morpholine, vibrational bands are assigned as symmetric or asymmetric stretch for polarized or depolarized bands, respectively.11 On the basis of the comparison between VSFG spectra and the Raman spectrum, the VSFG frequencies at 2791, 2826, 2855, 2942, and 2969 cm−1 can be assigned to symmetric stretching modes, and those at 2872 and 2916 cm−1 are expected to be asymmetric. However, this simple assignment of 2791 cm−1 to symmetric stretching mode is not in accord with the expected VSFG intensities. Symmetric 20135


Article


Figure 3. Comparison of OH stretching bands in VSFG spectra of air−water, air−morpholine, air−morpholine solution, air−HCl solution, and air− (morpholine + HCl) solution interfaces for SSP (a) and PPP (b) polarizations.

concentration increases. This statement is supported by measurement of the surface pressure, which shows an increasing trend with concentration of morpholine solution. However, the increase in surface pressure is gradual and small with the maximum value of about 2.5 mN/m at the highest concentration used. Since morpholine is highly soluble in water, the change in the surface pressure is small. Some contribution of a probable change in orientation of morpholine molecules at

the air−solution interface to the increase in SFG intensity cannot be ruled out. The orientation of morpholine molecules adsorbed on the surfaces of gold nanoparticles is reported to change from vertical at higher concentrations (≈100 mM) to flat at lower concentrations (≈10 mM) of morpholine solutions.6 In the CH2 stretching region, as can be seen from Figure 2, the VSFG spectra of the solution in both polarizations have 20136


Article


interface in comparison to that at the air−morpholine interface. The change in conformation can be due to different relative composition of equatorial and axial chair form or due to that of chair and boat/twisted boat form of morpholine. In the bulk liquid morpholine and its aqueous solution, the relative composition of equatorial and axial chair form is known to be different. The equatorial chair form has a preference in liquid morpholine, whereas the relative population of axial chair form increases in solution due to stronger hydrogen bonding of water molecules with NH of morpholine.15 In general, the calculated CH2 stretch frequencies for the axial chair form are observed at higher wavenumbers than that for the equatorial chair form of morpholine.6,11 Accordingly, in present work, the vibrational frequencies of the CH2 stretch of morpholine solution in bulk are found shifted to higher wavenumbers compared to liquid morpholine as detected by ATR-FTIR (shown in Table 1) indicating a higher population of the axial conformer. The CH2 stretch frequencies in the VSFG spectra of liquid morpholine, generated at the air−morpholine interface, are observed at higher or same wavenumbers as in ATR-FTIR spectra, generated at the bulk. In solution, the VSFG peaks are observed at even higher wavenumber or remain unaffected. This probably indicates that the axial chair form is more abundant than equatorial chair at the air−morpholine interface compared to bulk, and its relative abundance is still higher at the air−solution interfaces. These conclusions can be supported by the presence of vibrational bands 2791 and 2826 cm−1 (belonging to the equatorial chair conformer of morpholine11) in VSFG spectra with the SSP polarization at the air− morpholine interface, but absence of these bands at the air− solution interface (depicted in Figure 2). A higher fraction of the axial chair conformer at the air−solution interface can explain the enhanced intensity of the band at 2945 cm−1, which belongs to the axial chair conformer of morpholine.11 Additionally, morpholine molecules might have a boat/ twisted boat structure at the air−solution interface. Unlike the chair conformers, this conformer has both the polar centers, the NH bond and the O atom, oriented to the same side of the planar CCCC ring backbone as can be seen from its optimized structure (shown in Figure 1). Hence, on the surface, if lying flat, this conformer can have the unique energetically favorable alignment where both the polar parts stay inside the polar environment of bulk water, and the nonpolar CH bonds remain directed into the air. This alignment can have extra stabilization because of the polar−polar interaction of the water-NH/waterO regions, and can avoid the polar-non polar interaction for the water−CH region. A representation of such alignment is shown in Figure 1. A similar arrangement is not possible for the chair conformers, which have the two polar parts always at opposite sides of the planar CCCC ring. The boat conformer of interfacial morpholine molecules with flat orientation is expected to provide maximum number of CH2 groups pointing away from water to contribute the maximum to the C−H stretch along the normal to the interface. Thus, the boat conformer is expected to generate greater VSFG intensity with the SSP polarization than a chair conformer. Although our theoretical calculations suggest that the energy difference between the chair and twisted boat conformers at the B3LYP/6-311++G(d,p) level of theory in the SCI-PCM solvation model is reduced in solution to some extent, the former conformer still remains more stable. Thus, either of these two conformational changes in morpholine molecules at the air−solution interface can explain the observed VSFG

Figure 4. CH stretching bands in VSFG spectra of air−solution interface of different concentrations of morpholine in SSP polarization.

some noticeable differences from that of the pure liquid. First of all, we expected the peaks in the SSP spectra in the solutions of mM concentrations to be much weaker than those in pure liquid, since high solubility of morpholine in water will significantly reduce its concentration at the air−solution interface. However, the peaks in the SSP spectra in solution were much stronger than those in pure liquid, implying that the net polar orientation of morpholine molecules parallel to the surface normal is much higher at the air−solution interface compared to the pure liquid. This effect is caused mainly because of the interaction of morpholine molecules with surface water molecules. In addition, another important fact emerges when we compare the solution spectra with that of pure morpholine, as in Figure 2. The intensity of the SSP spectrum of the solution is higher than that of the PPP spectrum, whereas the spectral intensities in SSP and PPP polarizations are comparable in the pure morpholine. This reconfirms the inference that the extent of net orientation of the morpholine molecules on the surface in the direction parallel to the surface normal is much higher, implying narrower distribution of orientation in solution compared to pure liquid. There are significant differences between VSFG spectra of liquid morpholine and its solution at the air−morpholine and the air−solution interfaces, respectively with respect to peak positions. In the SSP spectra, seven vibrational bands were observed (as mentioned earlier) at 2791, 2826, 2855, 2872, 2916, 2942, and 2969 cm−1 for liquid morpholine, but only five bands were observed at 2854, 2883, 2926, 2945, and 2969 cm−1 for the solution. In solution, two bands at lower frequencies disappear, and others have enhanced VSFG intensities. Moreover, some stretch frequencies are shifted to higher wavenumbers. Like the VSFG spectrum of morpholine at the air−solution interface in the SSP, in the PPP polarization also some peaks vanish and others get modified (Figure 2 and Table 1). The band at 2793 cm−1 due to equatorial chair conformer at the air−morpholine interface vanishes at the air−solution interface, and the band at 2876 is blue-shifted to 2882 cm−1. On the basis of the above spectral results we can suggest that there is a significant change in the conformation and orientation of morpholine molecules at the air−solution 20137


Article


Figure 5. CH stretching frequency region in the VSFG spectra of air−HCl solution interface at different morpholine concentrations with SSP (a) and PPP (b) polarizations. Solid lines are fit to the experimental data.

assigned to the weaker coupling of the water molecule stretching modes, which is associated with a more disordered and asymmetric 4-coordinate (tetrahedral) hydrogen-bonding network. The strength of the peak, typical of liquid-like structure of the water surface, indicates the disorderliness.17,18 There is a peak at 3533 cm−1, too, which is barely observed in the SSP spectrum but quite prominent in the PPP spectrum. It is attributed to the coupling of the asymmetric stretch vibrational modes of OH bonds that are only weakly perturbed by hydrogen bonding to neighbors.19 Tarbucks et al. assigned these peaks as indication of organic molecules bonded to the water molecules on the uppermost surface.20 However, this peak also includes 3-coordinate water molecules that are double proton donor single proton acceptor (DDA) molecules. The narrow peak at 3700 cm−1 observed both in SSP and PPP polarization is assigned to the stretching of the free OH bond of water molecules that are directed into the air phase.17

spectral features. Further experimental studies and theoretical simulations are required to prove unambiguously whether a boat conformation resides on the surface of aqueous solution of morpholine. 3.3.B. Region between 3000 and 3900 cm−1. Before discussing the experimentally obtained VSFG spectrum of pure morpholine in this region, we present a short recollection of peaks of pure water in the 3000−3900 cm−1 region in order to understand our results better. In the SSP spectrum of pure water, there are three major peaks in the region of 3000−3800 cm−1, two broad ones centered at around 3250 and 3450 cm−1 and a sharp one at 3700 cm−1. Following generally accepted assignments for bulk and surface water, the 3250 cm−1 peak in the SSP spectrum is attributed to the strong intermolecular coupling of water molecule symmetric stretch vibrations within a symmetric hydrogen-bonding network. The strength of this peak indicates the orderliness in the water molecular arrangement.16−18 The peak at 3450 cm−1 in the SSP spectrum is 20138


Article


ionizes in aqueous environment, both morpholine and water molecules are capable of getting protonated. To understand how the orientation and structure of morpholine and water molecules change at the air−solution interface in presence of acid, we studied the effect of acid addition to morpholine solutions of different concentrations. First, 300 μL of concentrated HCl was added to a 10 mL volume of morpholine solution with concentrations of 10, 20, 30, 40, 50, and 60 mM and allowed to equilibrate for 12 h. The concentration of acid in these solutions was around 300 mM. The VSFG spectra of the solutions of different concentrations of morpholine in presence of HCl were measured at both polarizations, and compared to that of aqueous morpholine solution, pure water, pure morpholine, and 300 mM HCl solution. It was observed that the addition of HCl to pure water does not give any peak in the 2800−3000 cm−1 region, while in the 3000−3750 cm−1 region, it does alter the spectra of pure water. 3.4.A. Region between 2800 and 3000 cm−1. The SFG spectra of the CH2 vibrational modes of morpholine both in SSP and PPP polarizations undergo considerable change in both spectral position and relative intensity of peaks after HCl addition in morpholine-water solution (Figure 5, parts a and b). In general, the overall intensity of the peak heights decreases with increase in concentration of morpholine in both SSP and PPP polarizations. This effect of change in VSFG intensity is more obvious for the strongest band near 2950 cm−1. This trend is different from the case of morpholine solution in absence of HCl (Figure 4), wherein the VSFG intensities increase with morpholine bulk concentrations. In the SSP spectrum, the peak at 2851 cm−1 remains unchanged in all concentrations of morpholine, but the peak at 2880 cm−1 loses its intensity as morpholine concentration increases. As the morpholine concentration increases, however, in the SSP spectrum, a new peak builds up at 2932 cm−1, and the peak at 2953 cm−1 slowly loses its intensity. In the PPP spectrum, the peak at 2851 cm−1 increases with increasing morpholine concentration but the peaks at 2877, 2894, and 2933 cm−1 follow the opposite trend. The vibrational band observed at 2956 cm−1 at the air−solution interface is not seen at lower concentrations on addition of HCl, but with increasing morpholine concentration, it starts building up. However, the peak at 2971 cm−1 at the air−solution interface remains unchanged on addition of HCl and its intensity remains independent of morpholine concentration in presence of HCl. Earlier it was observed that the peak positions in the Raman spectrum of CH2 vibrational modes of morpholine changes significantly by addition of dilute acid.10 These changes were attributed to two probable effects, protonation of the N−H group, and change in the relative concentrations of different conformers due to favorable interactions with the surrounding environment. Both of these effects may also be responsible for the change in the SFG spectrum of morpholine solution in presence of HCl. The fact that we can see additional changes in the VSFG spectrum as morpholine concentration increases in presence of HCl of same strength, must be attributed to the change in the structure and orientation of morpholine molecules on the interface. In our experiments, concentration of HCl (300 mM) is always much higher than the morpholine concentration (10−60 mM), so all morpholine molecules are expected to get protonated. In such scenario, the distinction between the axial and equatorial chair conformers get obliterated, but the distinction between chair and boat conformers still exists. As predicted by the studies on surface

We measured the VSFG spectra of the aqueous solution of morpholine with both the SSP and PPP polarizations. The spectra of morpholine solution are compared with that of pure water and liquid morpholine. As can be seen from parts a and b of Figure 3, no signal could be detected that can be assigned to its N−H bond at the air−solution interface. Since the intensity of the N−H stretching peak is inherently weak, possibly it gets lost in the much stronger signal of water molecules on the surface of very dilute morpholine solution (millimolar level of concentration). The sharp peak at ∼3700 cm−1 is drastically reduced. Thus, most of the free OH bonds on the surface of water are engaged in hydrogen bonding with the morpholine molecules. In the SSP polarization, both the OH stretch modes due to hydrogen bonded water molecules at 3250 and 3450 cm−1 in the VSFG spectra get enhanced in the presence of morpholine (shown in the lower panel of Figure 3a), due to polar orientation of interfacial water molecules induced by hydrogen bonded interaction between morpholine and water molecules. The enhancement in the VSFG intensity increases with increasing bulk morpholine concentration. In the PPP spectrum of the 10−60 mM solution of morpholine, a single broad peak at 3555 cm−1 is observed. A typical spectrum of morpholine solution (40 mM) in comparison to pure water and liquid morpholine is shown in Figure 3b. This peak is around 20 cm−1 blue-shifted compared to the typical peak at 3530 cm−1 of pure water. Intensity of this peak increases with increasing bulk concentration of morpholine, indicating the role of increased surface concentration. This frequency also falls in the range of 3520−3650 cm−1, which includes OH stretching in an asymmetric environment,16 antisymmetric OH stretching,21 and water molecules with fewer hydrogen bonds, that give prominent OH stretches in cluster studies.22−24 Polarization dependent symmetry analysis attributed this broad 3550 cm−1 peak to the other OH group of the same water molecule with the “free OH” group at the topmost layer of the air/-water interface.25 High intensity peak in this region again justifies our inference drawn from the SSP spectrum of morpholine solution, that is, the morpholine molecules get hydrogen bonded to the “free OH” group of the water molecules at the interface and increase their net orientation. On the other hand, the disappearance of the “free OH” peak at 3700 cm−1 at all polarizations indicates that on the topmost surface, almost all interfacial water molecules are hydrogen bonded to morpholine molecules, leaving no free OH. These two observations together suggest that the morpholine molecules on the topmost surface engage the water molecules in hydrogen bonding where the O atom of H2O molecule can donate proton to both the N atom and the O atom of the morpholine molecule, and can accept proton from its N−H bond. Similar hydrogen bonding has been proposed in aqueous solution of piperazine, where the O atom of the morpholine ring is replaced by another N−H group. We have observed that in piperazine molecule, each N−H bond can participate in two hydrogen bonds with different H2O molecules in the aqueous solution.26 The N atom acts as a proton donor in the hydrogen bond parallel to the N−H bond and as a proton acceptor in the hydrogen bond vertical to it. 3.4. Effect of HCl. Multiple reports27,28 are available which discuss the effect of acid on the surface structure of water. Baldelli et al.28 carried out a detailed study on effect of HCl addition on water surface, with HCl concentration varying from 0.01 to 0.22 mol fraction. In presence of HCl, which completely 20139


Article


H+- Cl− bilayer formation.28 In presence of both morpholine and HCl, C4H9NHO+ cations, softer than protons, accumulate closer to the surface and thus decrease the distinction between anion−cation bilayer, which eventually brings down the ordering of hydrogen bonded network of water molecules. Presence of large number of charged centers also weakens the water−water H-bonding. At lower concentration of morpholine, this effect is much more prominent for decreasing the inherently weaker intensity OH stretching vibrations, compared to much intense CH stretching vibrations, hence the former is almost obliterated but the latter is still visible. As morpholine concentration increases, the effect becomes stronger even for CH stretching vibrations, as we have discussed in the previous section.

tension and surface potential of electrolytes and molecular simulations, the surface is deprived of small, hard ions compared to the bulk.29−31 Other studies have shown that the anions tend to accumulate at the surface whereas the cations are repelled from it.32,33 These two observations are supported by Baldelli et al. through their studies on the effect of dilute HCl on VSFG intensity of air−pure water surface.28 They suggested that at low concentration of HCl (0.01 mol fraction) aqueous solution, a subsurface electric double layer is formed where anions go up closer to the surface and the cations remain in a lower, subsurface layer.28 However, when HCl is added to morpholine solution, some protons are converted to much softer and bigger protonated morpholine (C4H9NHO+) ions, which have a higher tendency to accumulate nearer to the surface, and decrease the effect of the double layer. As concentration of morpholine increases, population of C4H9NHO+ cation increases. This causes them to come closer to form ion pairs C4H9NHO+-Cl− and thus overall charge separation near the surface decreases. This causes more randomization in the orientation of interfacial protonated morpholine molecules, bringing down the VSFG signal intensity of the CH stretching vibrations. 3.4.B. Region between 3000 and 3800 cm−1. The VSFG spectra of morpholine (in SSP and PPP polarizations) are shown alongside the other spectra in this region in parts a and b of Figure 3. As can be seen from these figures in the 3000− 3800 cm−1 region, the VSFG intensity due to hydrogen bonded H2O in aqueous solution of morpholine decreases both in SSP and PPP spectra on addition of HCl. In order to compare, we also recorded the SFG spectrum of only 300 mM HCl solution both in SSP and PPP polarization, in the range of 3000−3800 cm−1, and shown them in Figure 3, parts a and b. The recorded spectra showed that both HCl (300 mM) and morpholine (60 mM), when present individually, increases the intensity of all the peaks of hydrogen-bonded water (two broad peaks at 3250 cm−1and 3450 cm−1 in SSP polarization and a single broad one at 3533 cm−1 in PPP polarization). But, when they are present together, as in the solution of morpholine in 300 mM HCl, all these peaks are almost completely obliterated. In the SSP spectrum, a very weak and broad signal is visible roughly centered at 3615 cm−1. Richmond et al.20 have assigned this peak to water molecules in the topmost layer that interacts with organic molecules. This high frequency peak is not expected from highly coordinated OH species, but rather from species with few hydrogen bonds, i.e., symmetric OH stretching of weakly hydrogen bonded water molecules.16,22−24 The peak at 3700 cm−1 is also completely vanished, indicating negligible number of “free OH” groups on the surface. In the PPP polarization, a single strong broad peak is observed in aqueous solution of morpholine roughly centered at 3560 cm−1. This peak has been assigned to symmetric OH stretching of weakly hydrogen bonded water molecules (double donors).16,22−24 On addition of HCl, this peak is somewhat blue-shifted and reduced in intensity, which is identical with that of only HCl solution, as can be seen from Figure 3b. Since these peaks are much blue-shifted from that of pure water, (3533 cm−1), the hydrogen bonding among the interfacial water molecules is considered to become weaker as HCl is added to morpholine solution. As discussed in the previous section, the increase in OH signal intensity of water due to addition of HCl at low concentration has been explained by Baldelli et al. as effect of higher ordering induced on interfacial water molecules due to

4. CONCLUSIONS Vibrational sum-frequency generation (VSFG) spectroscopy was employed to investigate the structure and orientation of morpholine at the air−morpholine and air−solution interfaces. The CH stretch intensity in VSFG spectra due to interfacial morpholine molecules at the air−solution interface increases with increasing bulk concentration of morpholine solution, because of its increasing surface concentration. The CH stretch intensity is also enhanced at the air−solution interface in comparison to that at the air−morpholine interface. Similarly, the OH stretch intensity due to interfacial water molecules is enhanced at the air−solution interface. These results suggest increased polar orientation of interfacial morpholine and water molecules due to mainly hydrogen bonded interaction between them. In addition, the equatorial chair conformation of morpholine molecules at the air−morpholine interface changes to the axial chair or probably a twisted boat conformation at the air−solution interface contributing to the enhanced VSFG intensity in the CH region. However, the increased polar orientation of interfacial water molecules, induced by morpholine molecules, is lost on addition of dilute HCl solution, and it becomes random with almost absence of VSFG intensity in the OH stretch region. More experiments and/or molecular dynamics simulations are required to confirm whether or not morpholine molecules exist in the twisted boat conformation at the air−solution interface.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b05351. Theoretical background on vibrational sum-frequency generation and fitting parameters of Figures 2 and 4 (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*(A.K.) E-mail:[email protected]. Telephone:91 22 25590302. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to acknowledge the constant support and encouragement from Dr D.K. Palit, Head, Radiation and Photochemistry Division, BARC, and Dr B. N. Jagatap, Group Director, BARC, during this work. 20140


Article


■

(21) Fourkas, J. T.; walker, R. A.; Can, S. Z.; Gershgoren, E. Effects of Reorientation in Vibrational Sum-frequency Spectroscopy. J. Phys. Chem. C 2007, 111, 8902−8915. (22) Pribble, R. N.; Zwier, T. S. Size-specific Infrared Spectra of Benzene-(H2O)n Clusters (n = 1 through 7): Evidence for noncyclic (H2O)n structures. Science 1994, 265, 75−79. (23) Steinbach, C.; Andersson, P.; Kazimirski, J. K.; Buck, U.; Buch, V.; Beu, T. A. Infrared Predissociation Spectroscopy of Large Water Clusters: a Unique Probe of Cluster Surfaces. J. Phys. Chem. A 2004, 108, 6165−6174. (24) Andersson, P.; Steinbach, C.; Buck, U. Vibrational spectroscopy of large water clusters of known size. Eur. Phys. J. D 2003, D24, 53−56. (25) Feng, R.-R.; Guo, Y.; Wang, H.-F. Reorientation of the “Free OH” Group in the Top-most Layer of Air/Water Interface of Sodium Fluoride Aqueous Solution Probed with Sum-Frequency Generation Vibrational Spectroscopy. J. Chem. Phys. 2014, 141, 501−510. (26) SenGupta, S.; Maiti, N.; Chadha, R.; Kapoor, S. Probing of Different Conformations of Piperazine Using Raman Spectroscopy. Chem. Phys. 2014, 436−437, 55−62. (27) Mucha, M.; Frigato, T.; Levering, L. M.; Allen, H. C.; Tobias, D. J.; Dang, L. X.; Jungwirth, B. Enhanced Concentration of Polarizable Anions at the Liquid Water Surface: SHG Spectroscopy and MD Simulations of Sodium Thiocyanide. J. Phys. Chem. B 2005, 109, 7617−7623. (28) Baldelli, S.; Schnitzer, C.; Shultz, M. J. The Structure of Water on HCl Solutions Studied with Sum Frequency Generation. Chem. Phys. Lett. 1999, 302, 157−163. (29) Onsager, L.; Samaras, N. N. T. The Surface Tension of DebyeHuckel Electrolytes. J. Chem. Phys. 1934, 2, 528−536. (30) Dietter, J.; Morgner, H. Structure and Dynamics at the Surface of a Concentrated Aqueous Solution of CsF. Chem. Phys. 1997, 220, 261−278. (31) Langmuir, I. The Constitution and Fundamental Properties of Solids And Liquids. J. Am. Chem. Soc. 1917, 39, 1848−1906. (32) Jarvis, N. L.; Scheiman, M. A. Surface Potentials Of Aqueous Electrolyte Solutions. J. Phys. Chem. 1968, 72, 74−78. (33) Wilson, M. A.; Pohorille, A. Interaction of Monovalent Ions with the Water Liquid-Vapor Interface: a Molecular Dynamics Study. J. Chem. Phys. 1991, 95, 6005−6013.

REFERENCES

(1) Shen, Y. R. Surface Property Probed by Second-Harmonic and Sum-Frequency Generation. Nature 1989, 337, 519−525. (2) Lambert, A. G.; Davies, P. B.; Neivandt, D. J. Implementing the Theory of Sum Frequency Generation Vibrational Spectroscopy: A Tutorial Review. Appl. Spectrosc. Rev. 2005, 40, 103−114. (3) Oliver, T. A. A.; King, G. A.; Ashfold, M. N. R. The Conformer Resolved Ultraviolet Photodissociation of Morpholine. Chem. Sci. 2010, 1, 89−96. (4) Assaf, G.; Cansell, G.; Critcher, D.; Field, S.; Hayes, S.; Mathew, S.; Pettman, A. Application of a Process Friendly Morpholine Synthesis to (S,S)- Reboxetine. Tetrahedron Lett. 2010, 51, 5048− 5051. (5) Subramanian, A.; Gopalakrishnan, R.; Boopathi, C.; Balakrishnan, K.; Vasudevan, T.; Natesan, M.; Rengaswamy, N. S. Morpholine and Its Derivatives as Vapour Phase Corrosion Inhibitors in Mild Steel. Bull. Electrochem. 1998, 14, 289−290. (6) Xie, M.; Zhu, G.; Hu, Y.; Gu, H. Conformations of Morpholine in Liquid and Absorbed on Gold Nanoparticles Explored by Raman Spectroscopy and Theoretical Calculations. J. Phys. Chem. C 2011, 115, 20596−20602. (7) Vogel, V.; Shen, Y. R. Air/Liquid Interfaces and Adsorbed Molecular Monolayers Studied With Nonlinear Optical Technique. Annu. Rev. Mater. Sci. 1991, 21, 515−534. (8) Saha, A.; Upadhyaya, H. P.; Kumar, A.; Choudhury, S.; Naik, P. D. Sum Frequency Generation Spectroscopy of an Adsorbed Monolayer of Mixed Surfactants at an Air-water Interface. J. Phys. Chem. C 2014, 118, 3145−3155. (9) Capparelli, A. L.; Maranon, J.; Sorarrain, A. M.; Filgueria, R. R. A Theoretical Conformational Analysis of Morpholine. J. Mol. Struct. 1974, 23, 145−151. (10) Ashtekar, S.; Barrie, P. J.; Hargreaves, M.; Gladden, L. F. An FTRaman Study of the Template-Framework Interaction in AlPO4-Based Molecular Sieves. Angew. Chem., Int. Ed. Engl. 1997, 36, 876−878. (11) Vedal, D.; Ellestad, O. H.; Klaboe, P.; Hagen, G. The Vibrational Spectra of Piperidine and Morpholine and their Ndeuterated Analog. Spectrochim. Acta 1976, 32A, 877−890. (12) Lu, R.; Gan, W.; Wu, B.-h.; Zhang, Z.; Guo, Y.; Wang, H.-f C-H Stretching Vibrations of Methyl, Methylene, and Methine groups at the vapour/alcohol interfaces. J. Phys. Chem. B 2005, 109, 14118− 14129. (13) Lu, R.; Gan, W.; Wu, B.-h.; Chen, H.; Wang, H.-f. Vibrational Polarisation Spectroscopy of CH Stretching Modes of the Methylene Group at the Vapor/Liquid Interfaces with Sum Frequency Generation. J. Phys. Chem. B 2004, 108, 7297−7306. (14) Buchbinder, A. M.; Gibbs-Davis, J. M.; Stokes, G. Y.; Peterson, M. D.; Weitz, E.; Geiger, F. M. Method for Evaluating Vibrational Mode Assignments in Surface-Bound Cyclic Hydrocarbons Using Sum-Frequency Generation. J. Phys. Chem. C 2011, 115, 18284− 18294. (15) SenGupta, S.; Maiti, N.; Chadha, R.; Kapoor, S. Conformational Analysis of Morpholine Studied Using Raman Spectroscopy and Density Functional Theoretical Calculations. Chem. Phys. Lett. 2015, 639, 1−6. (16) Scherer, J. R. The Vibrational Spectroscopy of Water; Heyden: Philadelphia, 1978; Vol. 5. (17) Du, Q.; Superfine, R.; Freysz, E.; Shen, Y. R. Vibrational spectra of water at the vapor/water interface. Phys. Rev. Lett. 1993, 70, 2313− 2316. (18) Shen, Y. R.; Ostroverkhov, V. Sum-Frequency Vibrational Spectroscopy on Water Interfaces: Polar Orientation of Water Molecules at Interfaces. Chem. Rev. 2006, 106, 1140−1154. (19) Sung, J.; Kim, D. Motional effect in surface sum-frequency vibrational spectroscopy. J. Korean Phys. Soc. 2007, 51, 145−148. (20) Tarbuck, T. L.; Richmond, G. L. Adsorptions of Organosulfur Species at Aqueous Surfaces: Molecular Bonding and Orientation. J. Phys. Chem. B 2005, 109, 20868−20877. 20141


Vibrational Sum-Frequency Generation Study of Morpholine at Air

Recommend Documents