3.1 Theoretical: Geometries and Noncovalent Interactions.
Fig.1-Fig.6 are the optimized geometries of molecules, BA dimer and their complexes. The optimized geometry of benzene and ethanol was taken from Hema at al. It helps in establishing relationships among molecular structure and intermolecular interactions. Single point energy, interaction energy and dipole moment are summarised in Table 1.
The interaction between structural subunits modifies the bond length, bond angle or leads to some preferred geometry which is the most fundamental source to understand the intermolecular interaction between molecules. The possible interaction between molecules based on structural changes and interaction energies are discussed as:
Benzylamine (BA) (Fig.1) is a polar molecule (Table 1) and can interact with any polar molecule as well as non-polar molecules. BA molecules have a π-ring (π-plane) which interacts with H-doner molecules. Since such interaction is a plane-to-point kind of interaction, there could be more than one possible conformation geometries.
On dimerization of BA, either the NH2 group of BA molecules interact via H-bonding (Fig.2) or the N-H bond of one molecule takes a perpendicular position above the plane of π-ring of another molecule (Fig.3), with interaction energy -7.795 and -7.117 Kcal/mol respectively. In the H-bonding case, the bond length r(C-N) of the doner BA molecule gets increased by 0.005 Å while r(N-H) of the acceptor gets increased by 0.004 Å. In plane-to-point kind of interaction, the interaction distance between the N-H bond and π-ring of two BA molecules are 2.903 Å and 3.066 Å. Which is slightly higher than the Hp/π interaction distance of 2.635 Å. The N-H bond perpendicular to the π-ring get elongated as a result of interaction between two BA molecules. Hussain et al also reported that BA dimer is stabilized by NH/π and N-H------O type of molecular interactions.
BA and ethanol both are polar molecules and are capable of forming H-bond. As N-----H-O hydrogen bond is stronger than O----H-N hydrogen bond, it is expected that when BA-ethanol complex is formed (Fig.4), then they will interact via N----H-O bond (K.V. Zaitseva et al.  reported that aliphatic amines dissolved in methanol form O-H….N type H-bonding.). But π-ring of BA provides better stability to the O-H bond of ethanol, therefore, the O-H bond of ethanol takes a geometry perpendicular to π-ring at 2.930 Å. The CH2 group of BA provides flexibility to the N-H bond and so it interacts with ethanol via O----H-N hydrogen bonding with interaction energy -4.461 Kcal/mol and interaction length 2.220 Å. On clustering of BA and ethanol, the bond length of ethanol r(O-H) increases by 0.004 Å and r(O-C) increases by 0.006 Å while the bond length of BA r(N-H) increases by 0.004 Å and r(C-N) decreases by 0.008 Å.
The interaction between benzene and BA (Fig.5), results in geometry such that, the N-H bond of BA takes position perpendicular to π-ring of benzene at a distance of 3.056 Å. The dispersive forces between the π-π ring of benzene and BA cause the interaction of two π-rings in such a way that the two π-rings, one of benzene and other of BA, takes T-shape similar to benzene dimer (T-shape structure ) and the bond length r(C-H) of benzene gets decreased by 0.001 Å. Hobza et al  reported a similar kind of interaction between a C-H donor and a π-face of an aromatic moiety and characterized it by a shortening of the C–H bond length and a blue shift of the C–H stretching frequency and called it C–H...π interaction. The distance of the C-H bond of benzene and centre of π-ring of BA is 3.464 Å (larger than what is predicted by MP2 level theory 2.252 to 2.770 Å as a study by Kumar et al for adamantane–benzene complexes because the DFT method does not include the dispersive forces). So, the BA-benzene cluster is stabilized by two interactions, NH/π interaction and CH/π interaction with interaction energy -4.954 Kcal/mol.
In BA-ethanol-benzene complex (Fig.6), the complex is stabilized by a different kind of interaction such as NH/π interaction between BA-benzene, O-H…… N hydrogen bonding between BA-ethanol complex. The interaction energy of this complex is -13.36 Kcal/mol.
The intermolecular interactions are: (1) between BA dimer: NH/π and N-H----O type H-bonding interaction, (2) between benzene and BA: NH/π and CH/π and (3) between ethanol and BA: Hp/π and H-bonding. The space between benzene and benzylamine molecule is the largest.
3.2 FTIR spectral analysis:
FT-IR spectroscopy is a suitable technique to investigate the intermolecular interaction. The interaction between the different molecules leads to the formation of the new vibrational degree of freedom which appears at shifted frequencies. So, the vibrational frequency shifts, changes of shape and intensity of IR absorption peaks resulting from some characteristic functional groups can be attributed to the existence of intermolecular interaction. [29, 30]
The IR absorption bands are caused by changes between different vibrational states of bonds in molecules. Vibrational transitions can most easily be discussed based on a harmonic oscillator model in which Hooke's Law holds at least approximately. Using Hooke’s law, the vibrational frequency of a chemical bond, where atoms and the connecting bond are modelled as a simple harmonic oscillator, is -
Where k is force constant.
Within the Harmonic approximation, the IR absorption coefficient per unit length of a sample of volume V is[32, 33]
Where is the refractive index and
The intensity of absorption depends on the molecular dipole moment .
The frequency range of an IR absorption band, at which a given bond absorbs, depends on the strength of the bond and the masses that make the bond. According to Beer’s – Lambert’s law the increasing concentration of solution and increase in path length affect the intensity of the IR absorption band . So, the absorbance of a band is influenced by the number of molecules which undergo a change in their vibrational states upon absorption of IR radiation and the bond dipole moment.
Band assignment: The spectrum is evaluated by assigning vibrational peaks to different functional groups. In Table 2 the spectral peak and vibrational bands of pure components that were taken from the literature [35–38] are presented.
The experimental FTIR spectra of the mixture at different concentrations and of pure components (Fig.7) is a complicated one. So, the discussion of different frequency range depending upon the major vibrational bands is as follows:
O-H and N-H stretching vibration range: 3550-3100 cm-1
The measurement of the X-H stretching frequencies can be a preferred tool to investigate H-bond in X-H------Y system. According to Arunan et al , the greater the lengthening of the X-H bond in the X-H-------Y system, the stronger and shorter is the H------Y hydrogen bond and vice versa. H------Y hydrogen bond length also depends on the bond angle.
Both N-H and O-H stretching vibration lies in the spectral range 3550-3100 cm-1 (Fig.8). The O-H stretching vibration in ethanol shows a peak at 3308.63 cm-1 with transmittance 75.97 % while N-H stretching vibration in benzylamine show two peaks at 3371.10 cm-1 and 3284.83 cm-1 corresponding to symmetric and asymmetric (higher frequency) vibration. Benzene does not show any peak in this region. For x1=0.0, the peak is at 3324.93 cm-1 with increased transmittance to 90.57 %. There is a shift of 16.30 cm-1 toward a higher wavenumber. When benzene and ethanol are mixed, the H-bonding between ethanol molecules ruptures and the OH/π interaction between benzene and ethanol takes place. The shift in O-H stretching vibration wavenumber towards higher wavenumber shows that the O-H bond length of H-bonded ethanol (such as ethanol dimer) gets decreased on interacting with benzene via OH/π interaction and that the strength of OH/π interaction is weak compared to conventional H-bonding of ethanol. It is a well-established fact that the formation of hydrogen bonding lowers the frequency of OH stretching vibration and a broader band appears at a lower frequency as compared to the free OH group. This gives a sharp band in the frequency range of 3650-3590 cm-1 . The intensity of O-H stretching vibration get decreased at 0.0 mole fraction because compared to pure ethanol, a smaller number of ethanol molecules are present to absorb the radiation corresponding to O-H stretching vibration in the benzene-ethanol mixture. Previous, theoretical interaction energies also show that the OH/π interaction is weaker than H-bonding .
After 0.0 i.e. when BA is introduced in the mixture, the spectra become complex because O-H and N-H absorb in the same frequency range, but the N-H band is sharper and of lower intensity. The change in shape and shift in wavenumber take place with increasing concentration of BA. From Table 3, the respective peak position of N-H symmetric, N-H asymmetric and O-H stretching vibrations gets shifted. At x1= 0.6, the main peak of N-H stretching vibration gets shifted towards lower wavenumber i.e. shows redshift compared to the peak position in BA. This indicates that the N-H bond length gets increased in the BA-ethanol mixture as compared to pure BA. This results in an overall decrease in NH stretching vibration frequency.
At x1=0.6 (Fig.9), the deconvolution of the spectra shows that the peak in the 3500-3100 cm-1 is made of five components. One is due to O-H stretching (3360 cm-1), the second and third due to N-H symmetric and asymmetric stretching (3361.23 cm-1 and 3287.07 cm-1) and as the hydrogen bonding creates an additional relaxation channel, so the other two will be due to the interaction present in the mixture between different components such as OH/π and N-H----O interactions. Also, on mixing ethanol and BA, N-H stretching frequency gets red-shifted (Rajpurohit et al  show that N-H----O bonding shows a red-shift in amino group frequency.) while O-H stretching frequency gets blue shifted. This indicates that the strength of the interaction in the BA-eth mixture is weaker than that of pure ethanol but stronger than the interaction in pure BA interaction. On analysing the components of the deconvoluted spectrum, it is seen that on mixing ethanol and BA, the O-H stretching frequency get more affected compared to N-H vibration frequency indicating that in BA-ethanol mixture O-H------N hydrogen bonding dominates over the N-H-----O hydrogen bonding (Similar to aliphatic amine and alcohol mixture as reported by Zaitseva et al. ). Although, in the mixture both O-H------N and N-H-----O hydrogen bonding will be present. Theoretical results also predict the N-H------- O type hydrogen bonding in the BA-ethanol complex.
But with mole fraction, the intensity and wavenumber both do not follow a particular pattern because in a mixture the extent of interaction of each molecule with the other is different. The concentration of the mixture and relative strength of interactions are the key factor to decide the IR spectra of the mixture.
C-H stretching Vibration: 3100-2900 cm-1
The C-H stretching vibrations of ethanol, BA and benzene fall in the wavenumber range 3100-2900 cm-1 (Fig.10). For Benzene the peak is at 3035.86 cm-1. BA shows two C-H stretching vibration bands at 3026.06 & 2911.54 cm-1 due to the aromatic C-H and CH2 groups. Ethanol also shows two C-H stretching bands at 2973.24 and 2928.46 cm-1 due to CH2 and CH3 group (Table 4).
In aromatic C-H stretching vibration, the stabilization of π-π interaction in the polar environment (Diederich et al  reported that the strong π-π interaction in the polar solvent.) leads to the shortening of C-H bond length and causes an increase in aromatic C-H vibration frequency. This kind of vibrational frequency increase is also observed for ethanol- benzene at 0.0 mole fraction and BA-ethanol at 0.6 mole fraction. The increased polarity of solvent reduces the transition dipole moment of C-H vibration . This will cause a fall in the intensity of C-H vibration with increasing x1. The C-H stretching vibration frequency of ethanol gets decreased when it gets mixed with benzene (x1=0.0), indicating that the C-H bond length in the mixture gets decreased compared to H-bonded ethanol solvent. The increased amount of red-shift with mole fraction (Table 4) indicates the weakening of the C-H bond of ethanol.
When the scale in the wavelength range 3010-3045 cm-1 is expanded, it is observed that at various mole fractions (Fig.11), the aromatic C-H stretching vibration is found to be the combination of two aromatic C-H vibrations, one of benzene and other of BA. At lower mole fraction it is mainly due to benzene, with increasing mole fraction, the contribution of BA increases and at higher concentration, it is due to BA. At mid mole fraction, such as 0.3 the deconvolution of the band shows that the C-H vibration is due to benzene, BA, and the interaction between them (Fig.12).
Aromatic C-C stretching and NH2 bending Vibration: 1700-1550 cm-1
The mix band with peak at 1604.36 cm-1 and 1584.65 cm-1 corresponds to NH2 bending and aromatic C-C stretch of BA (Fig.13). The position of bands does not shift too much but their transmission gets decreased with increasing mole fraction. More the mole fraction, more BA molecule will be available to absorb the vibration.
Aromatic C-C stretching Vibration of benzene: 1465-1490 cm-1
The aromatic C-C vibration of benzene shows a peak at 1478.24 cm-1 with a transmittance of 80.26% (Fig.14) and shows a small shift toward a higher wavenumber with an increasing mole fraction (Table 5). The absorption intensity of IR spectra depends on molecular dipole moment[32, 33], so, the increased intensity of aromatic C-C stretching at 0.0 mole fraction represents that the molecular dipole moment gets increased in a polar (ethanol) environment. Polarizability is an important consideration for aromatic molecules for their interaction . This increased molecular dipole moment in polar solvent leads to strong π-π interaction as reported by Diederich et al . With further increase in mole fraction, the decreasing intensity is due to a decrease in benzene concentration. The strong π-π interaction leads to stabilization of π-ring which leads to a blue shift in absorption frequency. A similar observation of blue shift of ring C-C stretching frequency is reported by Li et al  for pyridine+ water complex.
C-N stretching vibration of BA and C-O stretching vibration of ethanol: 1070-1020 cm-1
The effect in the stretching frequency of C-N and C-O is due to the redistribution of electron density of molecules taking part in intermolecular interaction. This leads to variation in the bond length of the nearest neighbour of the atom that takes part in the interaction with other molecules as seen in theoretical calculation of optimized geometries.
From Fig.15, the position of the C-O stretching frequency of ethanol is 1046.03 cm-1. For all concentrations, the wavenumber of C-O stretching frequency gets increased compared to ethanol because, in all interactions involving ethanol (such as OH/π or N-H-----O) the C-O bond length gets increased. With increasing mole fraction, the C-O stretching frequency gets shifted towards a higher wavenumber. This is due to variation in the strength of interaction.
As we move from 0.0 to 0.1 mole fraction, the change in interaction from weak OH/π to strong H-bonding takes place which shows a considerable shift in frequency (from 1048 to 1049 cm-1). However, when the mole fraction is above 0.2, the nature of the interaction is the same so almost no shift in wavenumber is observed. The transmittance is the same for all mole fractions because the mole fraction of ethanol is fixed at all concentrations.
The C-N stretching vibrational band appears at 1025.41 cm-1 with a transmittance of 85.32 %. The C-N stretching vibration disappears at a lower mole fraction (lower than 0.5).