recently , considerable efforts are being made to design new noncentrosymmetric crystal structures by combining amino acids with various interesting organic and inorganic matrices to produce compounds for nonlinear optical ( nlo ) applications .
a number of l - histidine compounds exhibiting the nlo behaviour , namely , l - histidine acetate , l - histidine chloride monohydrate , l - histidine tetrafluoroborate , l - histidine hydrochloride monohydrate , l - histidine hydrofluoride dihydrate , l - histidine bromide , and l - histidinium trichloroacetate , were reported earlier .
the crystal growth and characterization of l - histidinium trifluoroacetate and l - histidine nitrate were reported from this laboratory [ 8 , 9 ] , recently . in this paper ,
another new compound possessing the nlo property , namely , l - histidinium 2-nitrobenzoate [ lh2nb , ( i ) ] is reported . to our knowledge ,
( i ) is the first reported compound of an amino acid with 2-nitrobenzoic acid .
the details regarding the preparation , crystal structure , hydrogen bonding , and shg efficiency of the title compound are discussed .
the starting compounds , namely , l - histidine ( loba chemie , 99% ) and 2-nitrobenzoic acid ( alfa aesar , 95% ) were used without further purification .
l - histidine and 2-nitrobenzoic acid were mixed in the stoichiometric ratio , in 1  :  1 proportions and dissolved in distilled water .
the resultant mixture was stirred continuously to obtain a homogeneous solution , filtered and kept undisturbed for crystallization to take place .
good quality single crystals of the title compound were obtained after about a week 's time .
three - dimensional intensity data for a crystal of ( i ) were collected on a bruker smart apex ccd area - detector diffractometer using graphite - monochromated mok radiation ( 0.71073   ) .
full - matrix least - squares refinement and subsequent fourier synthesis procedures were performed by using shelxl-97 .
the hydrogen atoms attached to the c atoms were positioned with idealized geometry and refined using a riding model [ c  h = 0.930.97   , uiso = 1.2  ueq ( parent c atom ) ] .
h bonding were located from difference fourier map and restrained to a distance of 0.86   . in the absence of significant anomalous scattering effects , friedel pairs ( 1130 ) were merged .
the crystal data , structure solution , and refinement parameters are listed in table 1 .
crystallographic data ( excluding structure factors ) for the structure of compound ( i ) reported in this paper have been deposited with the cambridge crystallographic data centre as supplementary publication no .
copies of the data can be obtained , free of charge , on application to ccdc , 12 union road , cambridge cb2 1 ez , uk ( fax : + 44-(0)1223 - 336033 or email : deposit@ccdc.cam.ac.uk ) .
a preliminary study of the powder shg was made with a laser beam of wavelength 1064  nm , using kurtz and perry technique .
the beam from a q switched nd : yag laser had an energy of 3.9  mj / pulse , pulse width of 8  ns , and the repetition rate being 10  hz . the crystals were ground to a uniform particle size of about 125150  m and then packed in capillaries of uniform bore and exposed to the laser radiation .
a powder of potassium dihydrogen phosphate ( kdp ) , with the same particle size , was used as a reference .
the output from the sample was monochromated to collect only the second harmonic (  = 532  nm ) , eliminating the fundamental , and the intensity was measured using a photomultiplier tube .
it was found that the shg conversion efficiency for the compound ( i ) is about two times that of the standard kdp crystals .
the molecular structure of the compound ( i ) with atom numbering scheme is shown in figure 2 .
the asymmetric part of the unit cell contains an l - histidinium cation and a nitrobenzoate anion .
the histidine molecule exists as histidinium ion due to the protonation at the n atom of the imidazole ring .
the 2-nitrobenzoic acid exists as nitrobenzoate since the proton gets transferred to the amino acid .
selected bond lengths , bond and torsion angles of ( i ) are listed in table 2 .
the amino and carboxylic groups of histidine are twisted by an angle ( c1c2c3c4 ) of 71.12 ( 2) ( table 2 ) , while this angle is 179.13(2) in the structure of l - histidine .
this type of rotation is a common feature in histidine - carboxylic acid structures [ 1 , 8 , 1315 ] .
the bond angles in the amino and carboxyl groups are normal as observed in the structure of l - histidine .
the plane of the carboxylate group of histidinium is twisted from its normal position , which may be due to the presence of the intermolecular n  ho bonds ( figure 3 ) from the nearby histidinium ions .
the nitro and carboxylate groups of the nitrobenzoate are twisted with respect to the planar benzene ring , as evident from the values of the torsion angles .
the planar six membered and five membered rings in nitrobenzoate and histidinium , respectively , are nearly perpendicular [ 82.99 ] to each other . excepting o6 , all the other oxygen atoms act as acceptors ( figure 3 ) in the formation of n  ho and c  ho hydrogen bonds ( table 3 )
the level of shg response of a material is inherently dependent upon its structural attributes . on a molecular scale ,
the extent of charge transfer ( ct ) across the nlo chromophore determines the level of shg output ; the greater the ct , the larger the shg output .
the presence of intermolecular interactions , such as hydrogen bonds , can extend this level of ct into the supramolecular realm , owing to their electrostatic and directed nature , thereby enhancing the shg response [ 17 , 18 ] . in lh2nb , the network of n 
hydrogen bonds running along the a - axis links the cationic histidinium ions ( figure 4(a ) ) .
further , the benzoates are interconnected by n  ho hydrogen bond to the histidinium molecule .
the interconnected molecules of lh2nb are stacked as arrays along the a - axis ( figures 4(a ) and 4(b ) ) and further stabilized by other n  ho bonds and weak interactions .  
the interplanar distances between the two six membered rings ( in 2-nitrobenzoates ) and the two five membered rings ( in the amino acids ) have the values of 3.480   and 3.307   , respectively , which fall in the category of - stacking .
such parallel and close stacking is very favorable for the promotion of ct through the lattice .
the large shg efficiency ( about two times that of the standard kdp ) of the compound ( i ) possibly arises due to ( i ) the large number of hydrogen bonds and ( ii ) the close stacking of histidinium cations and 2-nitrobenzozte anions , in the structure .
the crystals of a new nlo material from the amino acid family , namely , l - histidinium 2-nitrobenzoate ( lh2nb ) , were grown using slow evaporation technique .
the crystal structure of lh2nb was elucidated using single crystal x - ray diffraction methods .
the proton from 2-nitrobenzoic acid is transferred to the l - histidine , forming l - histidinium 2-nitrobenzoate .
the shg efficiency of this material was measured using kurtz and perry method and found to be about two times that of the standard kdp crystals .
the large number of hydrogen bonds , the - stacking of l - histidinium anions and 2-nitrobenzoate cations and the presence of strong electron acceptors and electron donors are , in part , the reasons for the large shg efficiency possessed by this material .
 a new nonlinear optical organic compound , namely , l - histidinium 2-nitrobenzoate ( abbreviated as lh2nb ( i ) ; ( [ c6h10n3o2]+ [ c7h4no4] ) ) , was synthesized . the molecular structure of lh2nb ( i ) was elucidated using single crystal x - ray diffraction technique . 
 the second harmonic generation ( shg ) efficiency of this compound is about two times that of the standard potassium dihydrogen phosphate crystals . 
1. Introduction
2. Experimental Procedures
3. Results and Discussions
4. Conclusions