1923
J. Phys. Chem. 1995, 99, 1923-1927
Nonlinear Optical Properties of Organic Molecules. 19. Calculations of the Structure, Electronic Properties, and Hyperpolarizabilities of Phenylsydnones John 0. Morley Chemistry Department, University College of Swansea, Singleton Park, Swansea, SA2 8PP, U.K. Received: September 20, 1994@
The structure and dipole moments of both 3- and 4-phenylsydnones along with derivatives containing both the dimethylamino and nitro groups have been calculated using the ab initio 3-21G basis set. The presence of the electron donor in the phenyl rings results in an increase in the magnitude of the dipole moment while the electron attractor has the opposite effect. The nonlinear optical properties of these structures have been assessed by calculating their molecular hyperpolarizabilities using a sum-over-states semiempirical approach. The results obtained show that the hyperpolarizabilities of the two isomers differ greatly with the dimethylamino group enhancing the value for the 3-phenyl derivative and the nitro group enhancing the value for the 4-phenyl derivative. The largest values are calculated for 4-phenylsydnones:
Introduction A considerable number of polar organic molecules have been assessed for application in nonlinear optics particularly in the areas of electrooptic modulation (EOM) and second harmonic generation (SHG).1-3 Effective materials generally contain donor and acceptor groups positioned at either end of a suitable conjugation path and typical examples are Nfl-dimethyl-4nitroaniline (la) and 4-(NJV-dimethylamino)-4'-nitrostilbene(11) where the magnitude of the effect increases with the increasing length of the conjugation path.
NO,
I1
I
H
I11
Iv
a, R' = R2 = R3 = H b,R1=Me2N;R2=R3=H C, R' = NO,; R2 = R3 = H d, R1= Me; R2 = R3 = H e, R1 = OEt; R2 = R3 = H f, R1 = Oipr; R2 = R3 = H g, R' = Br; R2 = R3 = H h, R'= NH2; Ri = R3 = H i, R' = R2 = H; R3 = Me
a,R=H b, R = Me2N C, R = NO2
SCHEME 1: Numbering Convention Adopted for the 3-Phenylsydnones (111)
\
a,R=Me b,R=H
/c10-c9\\
YJ 5
However, when active molecules such as Ia and I1 are processed into polymer films using a strong dc field for orientation near the glass transition temperature, the activity of the resulting cooled polymer film is much less than that expected from the molecular nonlinearity. This reduction arises in part from incomplete orientation in the electric field during the poling process coupled with a gradual molecular relaxation and reorientation afterward on cooling. Clearly, the strength of the poling field, the size of the active molecule, and the magnitude of the molecular dipole moment play an important role in the orientation process. Although both the aniline Ia and stilbene I1 have fairly large dipole moments of 6.84 and 6.83 D, respectively? more polar molecules with larger values would be expected to show enhanced behavior in the poling process provided they also possess substantial molecular hyperpolarizabilities. Mesoionic compounds such as the sydnones have fairly substantial dipole moments which would assist their orientation under poling conditions,but their nonlinear properties are unknown. The present studies have been directed at a study of both 3- and 4-phenylsydnones (I11 and IV, respectively) containing either a donor or an acceptor group in the phenyl rings. @
07
H6
I
Abstract published in Advance ACS Abstracts, January 1, 1995.
\ / X , 14-Cy
Y16
I
i""-"\"
/ \
/C8 -N3, C12-Cl3
SCHEME 2: Numbering Convention Adopted for the 4-Phenylsydnones (IV) 07 \\
C5-01
-cy
/X 14
Y16
/C8 -C4\ N3/ C12-CI3
I
H6
Methods of Calculation Molecular orbital calculations were carried out on empirical structures for all the sydnones (111and IV)using the 3-21G basis set5 of the GAMESS program6with full optimization of all bond lengths, angles, and torsion angles. The numbering convention adopted is shown in Schemes 1 and 2. The optimized structures were then used to calculate their molecular hyperpolarizabilities using the CNDOVSB m e t h ~ d ,a~ sum-over-states ,~ procedure (SOS) which has been specifically parameterised for both EOM and SHG applications. As in previous work, 50 excited states were used in the evaluation of the tensor, and all 27 components
0022-365419512099-1923$09.00/0 0 1995 American Chemical Society
Morley
1924 J. Phys. Chem., Vol. 99, No. 7, 1995 calculated by the CNDOVSB method in the Cartesian frame, though the most appropriate quantity for these studies is the vector component, 61, theoretically defined as7
itu
where /3l is aligned to lie along the direction of the molecular dipole moment (M). In addition to the frequency dependent value, a static value is also calculated in the absence of the applied frequency to give the quantity 60 which is an approximate measure of the intrinsic hyperpolarizabilityof a given molecular system. Discussion Structural Aspects. There is limited crystallographic information available on the structures of the substituted 3-phenyl(111) and 4-phenylsydnones (IV) discussed here with the exception of the former (111), where data are available on the 4'-methyl (IIId),9 4'-ethoxy (IIIe),9 4'-isopropoxy (IIIf),Io 4'bromo (IIIg),'I and 2'-amino (IIIh)'* derivatives. In these examples, the heterocyclic ring is essentially planar, but the phenyl ring is sometimes twisted around the N-phenyl bond. For example, while the phenyl ring is twisted by 36, 27, and 24" from the plane of the sydnone ring in the 4'-tolyl (IIId), 4'-bromophenyl (Ig), and the 4'-isopropoxyphenyl derivatives (1110, respectively, it is almost coplanar with the heterocyclic ring in the 4'-ethoxyphenyl- derivative (IIIe) (Table 1). However, in the case of the 2'-aminophenyl derivative (IIIh), there are two molecules present in the unit cell, one of which shows the phenyl ring broadly coplanar with the sydnone ring, while
in the other, the phenyl ring is twisted by 29" (Table 1). These results suggest that the barrier to rotation in the 3-phenylsydnones is very small, implying that there is little n-orbital overlap between the aromatic rings although the N3-C8 bond length at around 1.44 A would suggest otherwise (Table 1). Recent structural calculations on 3-(4-tolyl)sydnone (IIId)I3 using a variety of molecular orbital methods ranging from the semiempirical MNDO, AM1, and PM3 methods to ab initio methods with the STO-3G, 3-21G, and 6-31G** basis sets have shown that the 3-21G basis set gives the best fit with geometric data from the well-resolved crystal stru~ture.~ This method was adopted therefore in the current studies to calculate the structures of the parent sydnones (IIIa and IVa), and their derivatives containing the electron-donating dimethylamino group (IIIb and IVb) and the electron-attracting nitro group (IIIc and IVc). The results obtained for the 3-phenylsydnones (IIIa-IIIc) show a reasonable correlation with the crystallographic data though the 0 1 -N2 and C4-C5 distances are overestimated at 1.42 %, in each case (Table 2) versus the experimental values of 1.38 and around 1.40 A, respectively (Table 1). The calculated bond lengths of 1.32 and 1.33 A at N2-N3 and N3C4, however, show an excellent correlation with the experimental values of 1.31 and 1.33 A, respectively. All of the remaining bond lengths and angles show a very close correspondence with the experimental data. Surprisingly, the substituents appear to exert very little effect on the geometry in moving from either the parent (IIIa) to the 4'-dimethylamino derivative (IUb)or to the #-nitro derivative (IIIc). For example, neither the 01-N2 nor N2-N3 distance at 1.42 and 1.32 A, respectively, show any significant change in moving from the parent (IIIa) to either of the two derivatives IIIb or IIIc. Similar
TABLE 1: Crystallographic Data for 3-Phenylsydnones Taken from the Cambridge Structural Database parame teP 01-N2 N2-N3 N3-C4 c4-c5 C4-H6 C5-07 C5-01 N3-C8 C8-C9 C8-Cl3 C8-c10 c10-c11 Cll-c12 C12-Cl3 Cll-X14 0 1 -N2-N3 N2-N3-C4 N3-C4-C5 C4-C5-01 C5 -0 1-N2 C4-C5 -07 C5-C4-H6 C4-N3-C8 N3-C8-C9 C8-C9-C10 C9-C1O-C11 C1O-Cl1-C12 Cll-Cl2-Cl3 C12-C13-C8 C13-C8-C9 c10-c11-x14 C4-N3-C8-C9d R factor'
(11149 CIHGOP
(me19 CIHGIJ
( I I I DETGOX ~
(IIIgp BROPSY
1.384 1.314 1.329 1.404 0.995 1.217 1.408 1.445 1.375 1.379 1.379 1.378 1.379 1.378 1.493 103.0 115.7 106.6 103.3 111.4 136.5 127.8 127.7 119.5 118.2 121.9 118.4 121.2 118.8 121.5 121.4 35.9 0.031
1.385 1.307 1.329 1.402 1.061 1.211 1.407 1.447 1.370 1.380 1.392 1.385 1.379 1.376 1.362 103.5 115.3 106.8 103.4 110.9 136.9 130.0 127.8 119.6 119.3 119.4 120.4 120.3 119.0 121.6 124.9 -0.7 0.032
1.384 1.315 1.333 1.400 1.013 1.211 1.420 1.441 1.377 1.385 1.386 1.385 1.391 1.370 1.363 103.5 115.3 107.0 103.3 110.9 137.1 131.8 127.2 118.4 118.8 120.1 119.8 120.6 118.9 121.9 125.2 -23.7 0.041
1.365 1.336 1.379 1.411 1.081 1.201 1.421 1.407 1.410 1.402 1.366 1.423 1.378 1.378 1.889 104.2 114.6 104.8 105.1 111.3 135.7 128.5 127.2 118.8 120.5 119.0 120.5 120.9 119.0 120.4 118.0 26.9 0.082
(IIIh)'* APHSYDb 1.381 1.312 1.338 1.389 0.960 1.226 1.380 1.442 1.396 1.401 1.361 1.387 1.358 1.406 1.365 104.0 113.9 106.8 104.5 110.9 135.0 130.7 127.0 116.7 120.5 119.6 120.2 122.4 116.2 121.1 124.8' 6.4 0.062
1.386 1.312 1.321 1.394 0.826 1.218 1.385 1.444 1.391 1.407 1.376 1.379 1.369 1.412 1.354 103.5 114.3 107.5 103.6 111.1 135.9 129.0 126.9 116.9 120.1 118.9 121.4 121.9 115.4 122.4 125.0' -28.9 0.062
a Bond lengths are given in angstroms, angles in degrees. There are two molecules in the unit cell. R14 is joined at position 13 in this case (Scheme) with modified angle C8-C13-NH2. Clockwise rotation from the sydnone ring; positive values are below the ring plane. e A measure of the agreement between the structure as postulated relative to the diffraction data as collected.
NLO Optical Properties of Organic Molecules
J. Phys. Chem., Vol. 99,No. 7, 1995 1925
TABLE 2: Structural Data for Substituted 3-Phenylsydnones (III) and 4-Phenylsydnones (IV) Calculated at the 3-216Level variable" 01-N2 N2-N3 N3-C4 c4-c5 C5-01 C4-H6 N3 -H6 C5-07 N3-C8 C4-C8 C8-C9 C8-Cl3 C9-C10 c10-c11 Cll-c12 C12-C 13 Cll-X14 X14-Yl5 X14-Yl6 01-N2-N3 N2-N3-C4 N3-C4-C5 C4-C5-01 C5-01 -N2 C4-C5-07 C5-C4-H6 N2-N3 -H6 C4-N3-C8 C5-C4-C8 N3 -C8-C9 C4-C8-C9 C8-C9-C10 c9-c1o-c11 C1O-Cl1-C12 Cll-Cl2-Cl3 C12-Cl3-CS C13-C8-C9 c1o-c11-x14 c 1 l-xl4-Yl5 C1 l-Xl4-Yl6 C4-N3-CS-C9d C5-C4-CS-C9d ClO-Cll-Xl4-Yl5 C10-Cl l-Xl4-Yl6 dipole momente total energy'
(IIIa) 1.418 1.318 1.327 1.423 1.413 1.058
(1IIb)b 1.422 1.318 1.327 1.422 1.411 1.058
(IIIb)' 1.420 1.318 1.327 1.419 1.420 1.055
(IIIC) 1.414 1.320 1.326 1.424 1.416 1.058
1.200 1.437
1.201 1.429
1.202 1.431
1.198 1.430
1.381 1.382 1.381 1.383 1.385 1.383
102.7 115.7 107.6 102.6 111.3 134.9 128.5
1.380 1.382 1.373 1.404 1.405 1.374 1.366 1.461 1.461 102.7 115.7 107.7 102.8 111.2 134.8 128.7
1.380 1.384 1.372 1.403 1.405 1.373 1.366 1.462 1.462 103.0 115.2 107.9 102.8 111.0 134.7 127.9
1.382 1.382 1.377 1.376 1.377 1.379 1.449 1.243 1.242 102.5 116.0 107.5 102.5 111.4 134.9 128.5
126.2
126.2
126.6
126.2
118.9
119.9
120.2
118.8
119.3 120.1 119.9 120.5 118.8 121.4
120.3 121.1 117.3 121.4 120.0 119.9 121.2 120.5 120.5 30.9
120.7 121.2 117.0 121.5 120.2 119.4 121.3 120.3 120.4 1.1
119.3 119.0 122.0 119.3 118.8 121.7 118.9 117.1 117.1 30.8
-0.2 179.8 11.83
-0.4 179.7 11.94
-694.1 19
-694.118
-0.1 179.9 3.50 -764.053
30.8
8.88 -561.769
(IVa) 1.428 1.331 1.313 1.443 1.390
(IVb) 1.446 1.360 1.300 1.456 1.372
(IVC) 1.412 1.310 1.326 1.432 1.407
1.003 1.204
1.003 1.206
1.004 1.202
1.451 1.393 1.392 1.382 1.385 1.385 1.380
101.1 117.7 105.8 103.6 111.9 133.1
1.447 1.393 1.389 1.374 1.403 1.403 1.374 1.373 1.462 1.461 99.8 117.9 105.8 104.2 112.4 131.4
1.444 1.396 1.395 1.377 1.378 1.377 1.374 1.444 1.243 1.246 102.3 117.2 105.9 103.3 111.3 134.0
116.2
116.0
116.5
127.7
126.8
127.5
119.5 119.9 120.5 119.8 120.0 120.5 119.3
120.1 121.0 121.5 117.1 121.1 121.5 117.9 121.6 120.3 120.2
119.4 119.9 119.6 121.5 119.1 120.6 119.3 119.6 117.5 117.3
22.3
27.4 11.9 -167.2
7.04 -561.779
9.56 -694.126
13.6 0.6 - 179.4 5.61 -764.070
Bond lengths are given in angstroms, angles in degrees. Twisted conformer. Planar conformer. Clockwise rotation from the sydnone ring; positive values are below the ring plane. e In Debye. fIn au. a
trends are found in the experimental structures where the same bond lengths 0 1 -N2 and N2-N3 show fairly constant values of 1.38 and 1.31 A in the 4'-tolyl (IIId), 4'-ethoxyphenyl (IIIe), 4'-isopropoxy (IIIf), and 2'-amino (IIIh) derivatives (Table 1). The 4'-bromo derivative (IIIg) is an exception, however, but the experimental structure is less reliable as shown by the high R factor in this case (Table 1). In contrast, substituents present in the phenyl ring of 4-phenylsydnone (IV) are predicted to have a significant effect on the geometry of the heterocyclic ring (Table 2). Here, the 01-N2 and N2-N3 distances of 1.43 and 1.33 A respectively in the parent (IVa) increase to 1.45 and 1.36 A, respectively, in the 4'-dimethylamino derivative ( I n ) and decrease to 1.41 and 1.31 A, respectively, in the 4'-nitrosydnone (IVc). The calculated torsion angle of the phenyl ring in the 3-phenylsydnones (111) of around 31" in each of the three cases explored showed virtually no change from the starting position of the geometry optimization procedure (input value 30"). A repeat structure optimization on 3-(4'-(dimethy1amino)phenyl)sydnone (IIIb) with the phenyl ring now coplanar with the heterocyclic ring, gave a resulting planar structure (twist 1.1")
with almost identical bond lengths and angles to the twisted conformer (Table 2) though it is 0.77 kcal mol-' higher in energy. This result suggests that there is little to choose between the twisted and planar conformations for 3-phenylsydnones and helps to explain why the 4'-ethoxy derivative (IIIe) is planar and the 4'-isopropoxy derivative (IIIf) is twisted (Table 1). In the 4-phenylsydnones, the structure optimization at the 3-21G level produces different torsion angles for the phenyl ring depending on the nature of the substituents with the largest twist of 27" predicted for 4-(4'-(dimethy1amino)phenyl)sydnone ( I n ) and the smallest for the 4'-nitro derivative (IVc) at 14". In these cases, therefore, there appears to be a larger barrier to rotation than that found in the 3-phenylsydnones. Electronic Properties. The dipole moments of both 3-phenyl- (IIIa) and 4-phenylsydnone (IVa) at the 3-21G level of 8.88 and 7.04 D, respectively, increase with the introduction of the electron-donating dimethylamino group into the phenyl ring to 11.8 and 9.56 D, respectively, and decrease with the introduction of the nitro group to 3.50 and 5.61 D, respectively (Table 2). Almost identical trends and values are calculated by the
1926 J. Phys. Chem., Vol. 99, No. 7, 1995 TABLE 3: Calculated Hyperpolarizabilities and Excited-State Properties of the Phenylsydnones (111) and (IV) Obtained with the CNDOVSB Methodo R=O R = 1.17 molecule pg pe 1 f pl p2 pl p2 IIIab IIIb’ IIIbd IIIC IIId IVa IVb IVC
8.26 1.78 388 0.28 -0.48 0.36 -3.33 2.31 -0.41 11.4 5.16 377 0.30 10.5 0.38 20.2 11.6 4.70 388 0.26 12.7 0.39 25.8 -0.57 -4.78 2.85 9.40 413 0.19 -3.04 1.20 -8.10 -1.98 8.88 2.30 385 0.28 0.72 -0.26 -1.40 15.1 6.95 4.04 411 0.75 -6.08 5.41 -18.6 9.34 9.19 425 0.92 4.31 -5.47 11.6 -20.3 75.7 6.87 8.16 431 0.90 5.67 20.2 21.7 pexp, yp,and pe are the experimental and calculated ground- and excited-state dipole moments respectively (in debye); 1is the transition energy or absorption maximum (in nanometers); f is the oscillator strength; 8 is the frequency of the applied field (in electronvolts) used in the calculation; p l is the vector component of the molecular hyperpolarizability in the direction of the molecular dipole moment; p2 is the same quantity in the transverse direction (in units of cm5 esu-I). The experimental value is 6.48 D.4 Twisted conformer. Planar conformer.
TABLE 4: Calculated Atomic Charge at the Sydnone Ring and Cartesian Components of the Ground- and Excited-State DiDole MomenW ground state
first excited state
molecule p, 4 Q N.r Pi Q IIIa -8.00 -2.07 -0.154 -1.37 1.12 0.150 IIIb -11.0 -3.20 -0.173 -5.15 -0.28 0.056 IIIC -2.85 -0.08 -0.144 8.14 4.70 0.379 IVa -6.88 -1.02 -0.010 -3.85 -1.24 0.075 IVb -9.05 2.30 -0.056 -8.35 3.85 -0.121 IVC -2.75 0.032 1.53 -8.02 0.223 -6.29 “ y Xand y, are the dipole moments along the x and y coordinates (see text for reference frame); Q is the charge summed over all atoms in the sydnone ring only.
CNDOVSB method’ using the 3-21G structures (Table 3), but both methods appear to overestimate the experimental value for 3-phenylsydnone reported as 6.48 D.4 These results suggest that both derivatives IIIa and IVa are polarized from left to right with the sydnone ring behaving as a strong electron attractor. If the calculated atomic charges in 3-phenylsydnone (IIIa) at the CNDOVSB level are partitioned between the phenyl ring and the sydnone ring and summed over the atoms of each ring, there is an overall negative charge of -0.15 on the sydnone ring of the parent (IIIa) which increases to -0.17 with the presence of the dimethylamino group and decreases to -0.14 with the presence of the nitro group (Table 4). This change is reflected also in the sign of the components of the dipole moment and if the molecule (IIIa) is orientated with the ring nitrogen, N3, placed at the origin of the Cartesian coordinate system (O,O,O) and the ring oxygen 0 1 placed along the positive x direction (x,O,O) with the ring carbon, C4, forming the molecular plane, the major component of the dipole moment is in the x direction (px = -8.0 D) with the other major component in the transverse direction (uJ= -2.1 D, Table 4). The magnitude of the x component increases when the donor is present in the phenyl ring and decreases with the presence of the acceptor group (Table 4) as both substituents are approximately located in the direction of the x coordinate (-x,O,O). However, in the 4-phenylsydnones (IV) using the same Cartesian frame for comparative purposes, the position is more complicated because the donor and acceptor groups present in the phenyl ring are now positioned between the x and y coordinates. Thus, although the x component of the dipole moment of the 4-phenylsydnones (IV) increases and decreases with the presence of the donor and acceptor, respectively, as for the 3-phenyl-
Morley TABLE 5: Effect of the Number of Included Excited States on the Calculated Hyperpolarizability of 3-Phenylsydnone (1IIa)n
N
1
f
Pe
bl 17
1 4 5 6 8 9 10 15 16 20 30 39 40 50
388 29 1 257 250 22 1 218 215 194 193 178 153 141 139 120
0.28 0.11 0.50 0.24 0.38 0.23 0.13 0.17 0.21 0.01 0.01 0.15 0.00 0.00
1.78 5.41 13.5 9.22 6.06 6.64 8.47 11.2 11.5 4.22 1.64 3.51 4.91 20.3
-8.14 -5.28 - 1.42 - 1.40 -1.57 0.87 4.06 -2.23 -2.84 -3.03 -3.06 -3.57 -3.56 -3.33
ON is the number of the included excited state; 1 is the calculated transition energy (in nanometers); f is the oscillator strength; p, is the excited state dipole moment (in debye); and P I 17 is the vector component of the molecular hyperpolarizability in the direction of the molecular dipole moment at a field of 1.17 eV (in units of cm5 esu-I). sydnones (111),there is a substantial y component which becomes larger than the x component for the nitro derivative (IVc, Table 4). Calculated Hyperpolarizabilities. The hyperpolarizability of 3-phenylsydnone (IIIa) calculated with the CNDOVSB method’ either as the vector component along the dipole moment, pl (eq l), or as the vector component in the transverse direction, p2, is very small despite the very large reduction in the magnitude of the dipole moment from 8.26 to 1.78 D on excitation from the ground state to the first major excited state respectively (Table 3). This reduction is a result of a redistribution of electrons on excitation so that the overall charge in the sydnone ring changes from -0.15 in the ground state to 0.15 in the first excited state with a large reduction in the x component of the dipole moment and a reversal of sign of the y component (Table 4). The small hyperpolarizability obtained is surprising, as the large change in the dipole moment on excitation to the first excited state is similar to that observed for many polar organic systems where this state tends to dominate the overall value (the so-called two-state model).’-3 For example, the dipole moment of 4-nitroaniline (Ib) changes from 6.2 D in the ground state to 15 D in the first excited stateI4 and the hyperpolarizability is dominated by this single transition which contributes around 70% of its final v a 1 ~ e . l ~ A detailed analysis of the contribution of individual excited states to the hyperpolarizability of 3-phenylsydnone (IIIa), however, shows that there is a wide oscillation in its value with an increasing number of excited states (Table 5). Unlike the vast majority of donor-acceptor organic systems such as the aniline (Ia) or stilbene (11); therefore, the two-state model is no longer applicable for 3-phenylsydnone (IIIa), because there are a number of excited states which contribute to the final value of the hyperpolarizability. These predicted excited states which have significant oscillator strengths (Table 5), are also found experimentally. Thus there are three experimental absorptions found in the ultraviolet region at 310, 255, and 235 nm with extinction coefficients of 5650,7600, and 10 000 respectively,I6 which correspond to the calculated first, second, and fifth excited states (the third and fourth excited states have no oscillator strengths). The initial hyperpolarizability of the sydnone (IIIa) with only the first excited state included in the calculation, at -8.1 x cm5 esu-l, is gradually reduced with the inclusion of
J. Phys. Chem., Vol. 99, No. 7, 1995 1927
NLO Optical Properties of Organic Molecules further states, and after eight excited states have been included, the value falls to - 1.6 (Table 5). Thereafter, the value becomes positive with contributions from the ninth and tenth excited states, but then it reverts to a negative value after the 15th excited state. After this the hyperpolarizability value increases only slightly with the inclusion of further states and slowly converges with little change produced for the final 30 states (Table 5). The introduction of the electron-donating dimethylamino group into the phenyl ring, to give the lowest energy twisted conformer (IIIb), has a large effect on the hyperpolarizability with the vector component along the dipole moment, pl, dominant (Table 3). This value increases further for the fully planar conformer probably because of the greater donation of n-electrons from the donor to the sydnone acceptor. The corresponding 3-(4'-nitropheny1)sydnone (IIIc), however, has a much smaller value as a consequence of the two competing electron attractors located at both ends of the molecule, though the value is larger than the parent (IIIa). In both these cases, more than one excited state contribute to the final values. 4-Phenylsydnone (IVa) is calculated to have a much larger hyperpolarizability than the 3 isomer (IIIa) both for the vector component along the dipole moment, pl, and for the component in the transverse direction, p2 (Table 3). Again, a number of excited states contribute to the final value though in this case the first excited state has the strongest oscillator strength (Table 3). The introduction of the dimethylaminogroup into the phenyl ring surprisingly has little effect on the magnitude of the hyperpolarizabilitywhich is comparable to the case of the parent (IIIa, Table 3). Here the overall negative charge at the sydnone ring in the ground state increases in the first excited state, in contrast to the reversal of sign noted for the 3 isomer (Table 4). In contrast, the 4-nitro derivative (IVc) shows a large calculated value for the hyperpolarizabilitywhich is greater than that calculated for 3-(4'-dimethy1amino)sydnone (IIIb). The explanation for the dramatic differences between the substituted 3- and 4-phenylsydnones is not entirely clear but preferential stabilization of the resonance forms of the sydnone ring by the substituents is likely to be a contributory factor (Scheme 3). Thus the introduction of a donor into the 3-position of the sydnone ring will effectively stabilize the positive charge at that position while an attractor will have the opposite effect. Alternatively, the introduction of a attractor into the 4-position of the sydnone ring will effectively stabilize the negative charge at that position while a donor will have the opposite effect.
SCHEME 3: Resonance Forms of the Sydnone Ring
P-
H\c=c +I
R-No
\
,O
O
+I c-C\ //
R-No
N
,o
N
P-
H\c=c\ /
R-N,
,,o+
N
H
O
\c-C\ //
R-d,
,O+ N
Conclusions The 3-21G basis set appears to give a reasonable account of the geometries of substituted 3- and 4-phenylsydnones. The introduction of electron donors into the phenyl rings results in an increase in the magnitude of the dipole moment while electron attractors have the opposite effect. The hyperpolarizabilities of the two isomers differ greatly with the dimethylamino group enhancing the value for the 3-phenyl derivative and the nitro group enhancing the value for the 4-phenyl derivative. The largest values are calculated for 4-phenylsydnones. References and Notes (1) Nonlinear Optical Effects in Molecules and Polymers; Prasad, P. N., Williams, D. J., Eds.; John Wiley and Sons: New York, 1991. (2) Nonlinear Optics of Organics and Semiconductors; Kobayashi, K., Ed.; Springer-Verlag: Tokyo, 1989. (3) Nonlinear Optical Properties of Organic Molecules and Crystals; Chemla, D. S., Zyss, J., Eds.; Academic Press: New York, 1987. (4) McClellan, A. L.; Tables of Experimental Dipole Moments; W. H. Freeman: San Francisco, 1963, Vol. I; Rahara Enterprises: San Francisco, 1974, Vol. 11. (5) See for example: Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A.; Ab Initio Molecular Orbital Theory; John Wiley and Sons: New York, 1986. (6) Guest, M. F.; Shenvood, P.; GAMESS, an ab initio program; Daresbury Laboratory: Warrington, UK. (7) Docherty, V. J.; Pugh, D.; Morley, J. 0.;J . Chem. Soc., Faraday Trans. 2 1985, 81, 1179. (8) Morley, J. 0.;Pugh, D.; J. Chem. Soc., Faraday Trans. 2 1991, 87, 3021. (9) Wane. Y.: Lee. P. L.:. Yeh.. M.-Y.: Acta Crvstallogr. C ,ICr. Str. Comm.) 1984'40,'1226. (10) Uena, C.-H.; Lee, P. L.; Wana, Y.; Yeh, M.-Y.; Acta Crvstalloar. C (Cr. Str. eo".) 1985, 41, 1776. (1 1) Barnighausen, H.; Jellinek, K.; Munnik, J.; Vos, A,; Acta Crystallogr. 1963, 16, 471. (12) King, T. J.; Preston, P. N.; Suffolk, J. S.; Turnbull, K.; J. Chem. Soc., Perkin 2 1979, 1751. (13) Morley, J. 0. J. Chem. Soc., Perkin Trans. 2, in press. (14) Liptay, W. In Excited States; Lim, E. C., Ed.; Academic Press: New York, 1974; Vol. 1, p 129. (15) Pugh, D.; Morley, J. 0. Reference 3, Chapter 11-2, p 193. (16) Hammick, D. L.; Voaden, D. J.; J . Chem. Soc. 1961, 3303. JP942524P
