108
J. Binoy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 97–109
effects, which supply a clear experimental proof for the intermo-
lecular proton transfer mechanism.
lated geometry of M+Fꢁ shows that the non-planarity of melamin-
ium is caused by intermolecular hydrogen bonding with Fꢁ ion and
the crystalline environment. The bond order analysis and the com-
puted geometry reveal that the resonance structure observed for
the melamine and triazine rings are not present in M+ cation and
the loss of resonance is caused by intramolecular Hꢀ ꢀ ꢀH steric
repulsion between protonating hydrogen and the hydrogen atoms
of the amino group. The resonance structure is found to exist in the
CO bonds of formate ion, as predicted by DFT and XRD. The disrup-
tion of resonance structure of the ring on protonation of melamine
is found to cause elongation of bonds C2AN2 and N2AC3, providing
single bond character and is found to cause shortening of bonds
N1AC2 and N3AC3, providing double bond character. The resonance
structure is restricted to N3C1N1 moiety and consequent changes of
ring modes, upon protonation, has been noticed. The formate ion,
characterized by the resonance structure of the CO2 group, shows
shifting of vibrational spectral band positions due to intermolecu-
lar NAHꢀ ꢀ ꢀO interaction. The computed HOMO–LUMO energy gap
indicates an eventual charge transfer interaction in the molecule
and consequent hyperpolarizability enhancement, leading to NLO
activity 7.49, 5.59 and 3.15 times that of urea for M, M+ and
M+Fꢁ respectively. A new mechanism of proton transfer responsi-
ble for NLO activity has been suggested, based on anomalous IR
spectral bands in the high wavenumber region. The mechanism
of interaction between M+ and Fꢁ has been analyzed using molec-
ular electrostatic potential (MEP) mapping.
The FT-IR spectrum shows the spectral bands in the high wave-
number region, which indicate the NH group vibrations are af-
fected by surroundings strongly. The spectral bands observed at
2833 cmꢁ1 and 2765 cmꢁ1 are respectively ascribed to the stretch-
ing vibration of the hydroxyl and imine groups. The broad band at
2522 cmꢁ1 can be designated to the mixed O1ꢀ ꢀ ꢀH7 and O2ꢀ ꢀ ꢀH5
stretching mode, owing to the intermolecular hydrogen bonding
interaction, based on the previous literature [36].
Based on the detailed investigation on the hydrogen-bonding
spectral bands involving N and O atoms, we can further draw a
mechanism of the intermolecular proton transfer procedures for
M+Fꢁ [36]. Accompany with resonance, the intermolecular proton
transfereffectbringsoutthepositiveand negativemolecules (Fig. 9).
In most of the cases, even in the absence of inversion symmetry,
the strongest band in the Raman spectrum is weak in the IR-spec-
trum and vice versa. But the intramolecular charge from the donor
to acceptor group through a
p-bond conjugated path can include
large variations of both the molecular dipole moment and the
molecular polarizability, making IR and Raman activity strong at
the same time [37]. No such phenomenon can be observed for
M+Fꢁ and can be due to the absence of a prefect conjugated path,
which is distorted due to ‘partial resonance’. But ring distortion
band shows anomalous intensity in Raman for the band at
689 cmꢁ1, proving a possibility of charge transfer through ring
which has only partial orbital delocalization, in addition to the
charge transfer through intermolecular hydrogen bonding.
References
[1] M.P. Fernandez-Liencres, A. Navarro, J.J. Lopez-Gonzalez, M. Fernandez-Gomez,
J. Tomkinson, G.J. Kearley, Chem. Phys. 266 (2001) 1–17.
Analysis of MEP surface map
[2] M. Drozd, M.K. Marchewka, J. Mol. Struct. (Theochem) 716 (1–3) (2005) 175–
192.
Molecular electrostatic potential (MEP) function in space
[3] Paresh Chandra Ray, Chem. Phys. Lett. 394 (4-6) (2004) 354–360.
[4] C. Ma, H. Kang, Q. Liu, R. Zhu, Z. Cao, Toxicology 283 (2011) 96–100.
[5] Z.Q. Wen, G. Li, D. Ren, Appl. Spectrosc. 65 (2011) 514–521.
[6] L. Guo, J. Zhong, J. Wu, F. Fu, G. Chen, X. Zheng, S. Lin, Talanta 82 (2010) 1654–
1658.
[7] W.J. Jones, W.J. Orville-Thomas, Trans. Faraday. Soc. 55 (1959) 203–210.
[8] J. Binoy, I. Hubert Joe, V.S. Jayakumar, J. Raman Spectrosc. 36 (2005) 1091–
1100.
[9] J. Binoy, C. James, I. Hubert Joe, V.S. Jayakumar, J. Mol. Struct. 784 (2006) 32–
46.
[10] J. Binoy, Jose P. Abraham, I. Hubert Joe, V. George, V.S. Jayakumar, J. Aubard, O.
Faurskov Nielsen, J. Raman. Spectrosc. 36 (2005) 63–72.
[11] P. Hohenberg, W. Kohn, Phys. Rev. 136B (1964) 864–871.
[12] M.J. Frisch, G.W. Schlegel, H.B. Scuseria, G.E. Scuseria, M.A. Robb, J.R.
Cheeseman, J.A. Montgomery, T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam,
S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E.
Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E.
Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y.
Ayala, K. Morokuma, G.A. Voth, P. Salvadorm, J.J. Dannenberg, V.G. Zakrzewski,
S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.
Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.
Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A.
Pople, Gaussian ‘03, Revision C.02, Gaussian Inc., Wallingford, CT 06492, 2003.
[13] G.A. Petersson, M.A. Allaham, J. Chem. Phys. 94 (1991) 6081–6090.
[14] G.A. Petersson, A. Bennett, T.G. Tensfeldt, M.A. Allaham, W.A. Shirley, J.
Mantzaris, J. Chem. Phys. 89 (1988) 2193–2218.
around a molecule can be expressed as
Z
0
X
~
Zi
q
ðr Þ
0
~
~
VðrÞ ¼
ꢁ
dr
0
~ ~
~
jr ꢁ r j
~
jRi ꢁ rj
i
~
where Zi is the charge of the nucleus, Ri is the location of the nucleus
0
~
and
q
ðr Þ is the electron density function of the molecule. The first
term is the potential due to nucleus, second term is the potential
~
due to electron and VðrÞ is the resultant. The molecular electrostatic
potential serves as a useful quantity to explain hydrogen bonding,
reactivity and structure activity relationship of biomolecules and
drugs [38].
The electrostatic contour maps have been plotted (Fig. 10) over
the optimized geometry of stable isomers of M, M+, Fꢁ and M+Fꢁ
using B3LYP/6-31G(d) basis set. The different values of potentials
are represented by different colors and the color gradient ranges
from most electronegative red (ꢁ0.0500 eV) to the most electro-
positive white (+0.0500 eV).
The analysis of contour maps reveals that for melamine, the
most electronegative regions are the ring nitrogen atoms with no
substituents. On protonation, the electrophilic sites are shifted to
the amino groups attached to the ring and for Fꢁ, the two oxygen
atoms are found to serve as the reactive sites for the electrophilic
attack. Formation of M+Fꢁ is followed by the shift of electron den-
sity from amino group to the nitrogen atoms of the ring. The pro-
tonating hydrogen and amino group hydrogen act as the centers of
nucleophilic attack, thereby favoring the formation of intermolec-
ular hydrogen bonding in M+Fꢁ.
[15] J.B. Foresman, A. Frisch, Exploring Chemistry with Electronic Structure
Methods, second ed., Gaussian, Pittsburgh, PA, 1996.
[16] M. Alcolea Palafox, Int. J. Quantum. Chem. 77 (2000) 661–684.
[17] M. Alcolea Palafox, M. Gill, N.J. Nunez, V.K. Rastogi, Lalit Mittal, Rekha Sharma,
Int. J. Quantum. Chem. 103 (2005) 394–421.
[18] E.D. Glendening, A.E. Reed, J.E. Carpenter, F. Weinhold, NBO Version 3.1.
[19] R.F.W. Bader, Atoms in Molecules: A Quantum Theory, Oxford University Press,
Oxford, 1990.
Conclusion
[20] Genivaldo Julio Perpe´tuo, Marcos Antoˆnio Ribeiro, Jan Janczak, Acta. Cryst. 61E
(2005) o1818–o1820.
´
[21] J. Janczak, G.J. Perpetuo, Acta. Cryst. 58C (2002) o455–o459.
The FT IR and FT Raman spectral analysis, aided by DFT com-
puted spectrum and PED, has been applied to investigate bonding
and structural features of melaminium formate crystal. The simu-
[22] J. Janczak, G.J. Perpe´tuo, Acta. Cryst. 59C (2003) o349–o352.
[23] J.N. Varghese, A.M. O’Connel, E.N. Maslen, Acta. Cryst. 33B (1977) 2102–2108.
[24] J. Janczak, G.J. Perpe´tuo, Acta. Cryst. 60C (2004) o211–o214.
Binoy
