Effect of Electric Field on ANTA

The present study considers an insensitive explosive, ANTA, (5(3)-amino-3(5)-nitro-1H-1,2,4-triazole) which is optimized within the restrictions of DFT (B3LYP/cc-PVTZ and B3LYP/6-311++G(d,p)). The optimized structure is subsequently subjected to single-point semi empirical MNDO and/or PM3 level of calculations to visualize the effect of electric field which has been set to magnitudes of 0.001 and 0.01 au. Perturbations on the energy and dipole moment are investigated. Also, the directional effect of the field along the axes of inertia of the molecule has been investigated. Generally, the effect of the field is more pronounced along the principle axis of ANTA.


Introduction
Among various molecular properties, molecular hyperpolarizabilities, namely β and γ have attracted special attention of scientists for couple of decades. When light is incident on a material, the optical electric field, E, results in a polarization P of the material. The polarization can be expressed as the sum of the linear polarization component, P L and nonlinear polarization P NL [1][2][3][4][5]. P = P L + P NL (1) P L = χ (1) .E (2) P NL = χ (2) .EE + χ (3) .EEE +....
The susceptibility tensors χ (n) give the correct relationship for the macroscopic material. For individual molecules, the polarizability α, hyperpolarizability β, and second Lemi Türker http://www.earthlinepublishers.com 330 hyperpolarizability γ are defined as certain tensor quantities. The susceptibility tensors are weight averages of the molecular values.
In the present study, ANTA molecule has been subjected to electric field of different strengths and the resultant polarization effects have been investigated.

Method of Calculation
The initial geometry optimizations of all the structures leading to energy minima were achieved by using MM2 method followed by semi-empirical PM3 self-consistent fields molecular orbital (SCF MO) method [18,19] at the restricted level [20]. Then, the structure optimizations have been achieved within the framework of Hartree-Fock (HF) and finally by using density functional theory (DFT) at the levels of B3LYP/CC-PVTZ The calculations related to polarizability have been done by means of single point semi-empirical MNDO and PM3 type calculations based on the optimized structures obtained by density functional approach at the levels of B3LYP/cc-PVTZ and B3LYP/6-311++G(d,p). The directional effect of electric field along the axes of inertia of ANTA molecule have been searched by using Hyperchem 06 program [27].

Results and Discussion
Asymmetric substitution of a conjugated chain with a donor group on one end and an acceptor group on the other end provides the noncenterosymmetry required for a secondorder nonlinearity [28][29][30][31][32][33]. Since, only the time averaged asymmetrically induced polarization leads to second order nonlinear optical (NLO) effects, only molecules and materials lacking a center of symmetry can exhibit them [28]. The assumption is frequently made that the atoms or molecules are independently polarized by the light with no interatomic or intermolecular coupling [28]. ANTA molecule fulfils that condition. The optimized structure belongs to C1 point group (at both levels of calculations). Figure  1 shows the optimized structure of ANTA (B3LYP/6-311++G(d,p)). The B3LYP/cc-PVTZ level of calculation also leads to very similar optimized structure. Nonlinear polarization becomes more important with increasing field strength. Under normal conditions αE>βE>γE where α, β and γ refer to polarizability, the first and second order polarizabilities, respectively.
When an electric field (EF) is applied, the electron distribution and molecular geometry are disturbed. The atomic polarizability arises due to geometrical disturbance. It is significantly smaller than the electronic polarizability. Note that orientation polarizability arises when the molecules have a permanent dipole moment. ANTA has the dipole moment of 8.42 debye (B3LYP/6 respond strongly to the application of the field centroid of negative charge is displaced [3].   (   Tables 1A and 1B show the magnitude of dipole moment components as the polarization value increases ten times. The results indicate that the resultant magnitudes decrease as the polarization increases. This behavior holds for every level of calculations presented in the Tables components show parallelism in sign irrespective of the kind of method but when the polarizability value is increased to P:0.01 that property is lost, especially in the z component. polarizability arises when the molecules have a permanent dipole moment. ANTA has the ent of 8.42 debye (B3LYP/6-311++G(d,p)). Highly polarizable molecules respond strongly to the application of the field [3]. They become highly polarized and the centroid of negative charge is displaced [3].
Optimized structure of ANTA (B3LYP/6-311++G(d,p)). Figure 2 shows the bond lengths of ANTA molecule at two different level of calculations. The lengths differ only very slightly at two different level of calculations the present study value of the applied field is kept rather low just to prevent any disturbances of the molecular geometry.
Bond lengths (Å) in ANTA. A: B3LYP/cc-PVTZ, B: B3LYP/6 Tables 1A and 1B show the magnitude of dipole moment components as the polarization value increases ten times. The results indicate that the resultant magnitudes decrease as the polarization increases. This behavior holds for every level of calculations Tables 1A and 1B. Note that in the case of P:0.001 the individual components show parallelism in sign irrespective of the kind of method but when the polarizability value is increased to P:0.01 that property is lost, especially in the z Lemi Türker polarizability arises when the molecules have a permanent dipole moment. ANTA has the 311++G(d,p)). Highly polarizable molecules [3]. They become highly polarized and the 311++G(d,p)). Tables 1A and 1B show the magnitude of dipole moment components as the polarization value increases ten times. The results indicate that the resultant magnitudes decrease as the polarization increases. This behavior holds for every level of calculations 1A and 1B. Note that in the case of P:0.001 the individual components show parallelism in sign irrespective of the kind of method but when the polarizability value is increased to P:0.01 that property is lost, especially in the z- The results of calculations related to polarizability of ANTA molecule are included in Tables 1-5.  If the molecule is considered to be in a uniform electric field aligned along one of the axis of the system, the values of the polarizabilities along that axis (μ x , α xx , β xxx , γ xxx ) can be obtained by using eq.4. Tables 2A and 2B list the components of polarizability α, where E4 and dipole are the contributors of polarizability to energy and dipole moment, respectively. The results show that the average polarizability increases as the polarization value increases. Note that the diagonal elements exhibit parallelism in sign irrespective of the method applied but it does not hold for the off-diagonal elements.  The tensor components of the second order polarizability (β) are presented in Tables  3A and 3B.  The vector components of β and its value in the dipole moment are presented in Tables 4A and 4B. As seen in Tables 4A and 4B, the value of β in dipole moment increases as the polarizability value increased ten times. Also note that in each method applied, the value of β increases as the polarization increases. In the table components BI are expressed by the formula [2], In au, esu units in parenthesis. In au, esu units in parenthesis.
Tables 5A and 5B show the third order polarizability (γ) vector components involved in the calculation of averaged γ values as the applied polarizability value increased. The average value of γ is expressed as [2], Average Gamma=(1/5) [XXXX + YYYY + ZZZZ + 2 ( XXYY + XXZZ + YYZZ)] (10) As seen in the tables γ strongly responds to the increase of polarizability.  In au units. esu units in parenthesis. Figure 3 shows the axes of inertia of ANTA molecule. In the figure, X-axis is along the longest principle axis of the molecule whereas Y axis is perpendicular to X-axis. The Z-axis is perpendicular to XY-plane.  Table 6 shows the effect of field applied along the axes of ANTA molecule and strength on the dipole and its components. The components of the dipole are designated as x, y and z. The calculations have been performed at the level of MNDO//B3LYP/cc-PVTZ.   Table 6 shows the effect of field applied along the axes of inertia (X,Y and Z) of ANTA molecule and strength on the dipole and its components. The components of the dipole are designated as x, y and z. The calculations have been performed at the level of PVTZ.
. Effect of field direction and strength on dipole and its components of ANTA As seen in Table 6, the total value of the dipole moment increases if the field strength (F) is increased in the direction of the X (NO 2 ) groups reside. The way that the value of dipole moment vector increases as the field strength is increased. However, increase of the field strength along the Y or Z axes slightly decreases the dipole moment value.  As seen in Table 6, the total value of the dipole moment increases if the field strength (F) is increased in the direction of the X-axis along which the donor (NH ) groups reside. The applied electric field perturbs the electron population in such a way that the value of dipole moment vector increases as the field strength is increased. However, increase of the field strength along the Y or Z axes slightly decreases the dipole Figure 4 displays the effect of electric field on the molecular orbital energy spectra of Effect of electric field on the molecular orbital energy spectra of ANTA field in au units, MNDO// B3LYP/cc-PVTZ)).
As seen in Table 6, the total value of the dipole moment increases if the field strength axis along which the donor (NH 2 ) and acceptor applied electric field perturbs the electron population in such a way that the value of dipole moment vector increases as the field strength is increased. However, increase of the field strength along the Y or Z axes slightly decreases the dipole Figure 4 displays the effect of electric field on the molecular orbital energy spectra of Effect of electric field on the molecular orbital energy spectra of ANTA F Z =0.01 340 ANTA molecule. In the case of F: 0.001 the distribution of molecular orbital energy levels does not exhibit discernable perturbation depending on the field direction. Whereas when F value is set to 0.01 au, the distribution standing for F x case is different from the others. Table 7 shows the effect of field strength on the HOMO, LUMO energies and the interfrontier molecular orbital energy gap (∆ε) value of ANTA molecule. Note that ∆ε = ε LUMOε HOMO . As seen in the table, as the field in X, Y and Z-directions increases the HOMO energy level raises up. Again the greatest effect occurs in case of X-direction. On the contrary, the LUMO energy decreases as the field strength increases in X, Y and Zdirections in all the cases. Consequently, the interfrontier molecular orbital energy gap value decreases as the field strength increases, keeping the direction be the same. The electron distribution can be distorted readily if the LUMO energy level lies close to the HOMO energy, so the polarizability is then large. If the LUMO lies high above the HOMO, an applied field cannot perturb the electron distribution significantly and the polarizability is low [34]. Note that the impact sensitivity of an explosive is related to Δε value such that decrease of it increases the sensitivity [35,36]. Also note that in the case of ANTA the decrease in Δε is greater if the field direction lies along the X-axis. The first and second entries in each row have E values of 0.001 and 0.01 in eV, respectively. Figures 5 and 6 show the HOMO and LUMO patterns of ANTA molecule in two different field strengths. As seen in occurs on the LUMO pattern along the X-and Z-axes.

Conclusion
The present study on ANTA molecule has revealed that the applied electric field mainly affects ANTA when the direction of the field is set along the principle axis of the molecule. Then the HOMO energy level raises up w Meanwhile the interfrontier molecular orbital energy gap decreases as the magnitude of the field increases. So, the impact sensitivity of ANTA may increase by exposing it to an electric field. Obviously, http://www.earthlinepublishers.com The HOMO and LUMO patterns of ANTA in F: 0.01 au. (MNDO//B3LYP/cc The present study on ANTA molecule has revealed that the applied electric field mainly affects ANTA when the direction of the field is set along the principle axis of the molecule. Then the HOMO energy level raises up whereas the LUMO level decreases. Meanwhile the interfrontier molecular orbital energy gap decreases as the magnitude of the field increases. So, the impact sensitivity of ANTA may increase by exposing it to an electric field. Obviously, ab initio methods are expected to yield reliable and The present study on ANTA molecule has revealed that the applied electric field mainly affects ANTA when the direction of the field is set along the principle axis of the hereas the LUMO level decreases. Meanwhile the interfrontier molecular orbital energy gap decreases as the magnitude of the field increases. So, the impact sensitivity of ANTA may increase by exposing it to an re expected to yield reliable and quantitatively correct results. Note that semi empirical calculations tend to be qualitative unless the structure optimization is done by means of ab initio/DFT methods at high level basis sets.