Conformational stabilization of phthalan: physical origin of tiny ring-puckering barrier

Journal of Molecular Structure 609 (2002) 159±167

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Conformational stabilization of phthalan: physical origin of tiny ring-puckering barrier Sangmi Jeon a, Jaebum Choo a,*, Sungwhan Kim b, Younghi Kwon b, Jin-Yeol Kim b, Young-Il Lee c, Hoeil Chung d a

Department of Chemistry, Hanyang University, Ansan 425-791, South Korea Department of Chemistry, Hanyang University, Seoul 133-791, South Korea c Dongbu Research Council, Daejeon 305-708, South Korea d ABB Bomem Ltd, Seoul 135-090, South Korea

b

Received 29 October 2001; revised 30 November 2001; accepted 30 November 2001

Abstract The conformational property of phthalan has been investigated using ab initio calculation and natural bond orbital (NBO) analysis methods. Geometry optimizations for the planar (C2v) and puckered (Cs) conformers have been carried out using the HF, B3LYP, and MP methods, and the results indicate that this molecule has a tiny ring-puckering barrier. This barrier appears to be in good agreement with the previous experimental result. NBO analysis shows that the tiny ring-puckering barrier is closely related to the molecular orbital interactions around the C±O bonds of the ®ve-membered ring. The gas-phase infrared and liquid-phase Raman spectra of phthalan and 1,3-benzodioxole have been recorded and analyzed in terms of C2v symmetry. Vibrational frequency calculations using the B3LYP method have also been performed to compare with the spectroscopic data. The B3LYP frequency calculations do a reasonable job of estimating the frequencies. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Phthalan; 1,3-Benzodioxole; Vibrational spectroscopy; Natural bond analysis; Ring-puckering potential energy function

1. Introduction The conformational structure of phthalan (PHT) has been the subject of continuing interest, particularly with regard to its planarity. There have been several reports concerning the conformational structure of PHT. Smithson et al. [1] reported that the equilibrium structure of PHT is planar based on their far-infrared investigation. On the contrary, Hassan and Hollas [2,3] claimed the non-planarity * Corresponding author. Tel.: 182-31400-5505; fax: 182-314073863. E-mail address: [email protected] (J. Choo).

of the molecule from the single vibronic level ¯uorescence (SVLF) study. They suggested that nonplanarity is attributed to a twisting motion of the methylene carbons of the ®ve-membered ring. Caminati et al. [4] showed that PHT appears to be nonplanar with a tiny ring-puckering barrier of 40 cm 21 from the analysis of their rotational spectrum. More recently, Laane and co-workers [5±8] determined a two-dimensional potential energy surface in terms of ring-puckering and ring-¯apping co-ordinates from the far-infrared and Raman spectra of PHT. According to their two-dimensional potential energy surface of PHT, the barrier to planarity of this molecule is 35 cm 21. On the basis of those experimental

0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(01)00968-1

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results, it is believed that PHT has a tiny ring-puckering barrier.

In order to compare the recent experimental results with calculations, a theoretical investigation should be performed. As a step toward a better understanding of the conformational features of PHT, we have undertaken a computational investigation of this molecule using ab initio calculation and natural bond orbital (NBO) analysis. The planar (C2v) and puckered (Cs) structures of PHT have been optimized using HF, B3LYP and MP2 levels with the 6-31G(d) basis set, and then high level electron correlation treatments with extended basis sets have been carried out to predict a reliable puckering barrier. In addition, the NBO analysis has also been carried out to understand the orbital interactions that determine the conformational preference of PHT. Recently, we performed the NBO analysis on the optimized structure of 1,3benzodioxole (BZD) to understand the orbital interactions that determine the conformational preference of the molecule [9,10]. In this work, the NBO analysis of the Hartree±Fock wavefunctions shows that the interaction between the oxygen lone pair (np) and p the sCO orbital, which is closely associated with the anomeric effect, is the most important factor favoring the non-planar conformation. These results strongly support the conclusion which was drawn from the spectroscopic work [11,12]. In the present work, the NBO study has been extended to PHT in order to ®gure out the physical origin of the tiny ring-puckering barrier for the molecule. The conformational energies around the C±O bonds can be decomposed using the NBO method, which allows the calculation of the hyperconjugative energy contributions to the total conformational energy. In this way, one obtains the main orbital interactions to contribute to the conformational stabilization of PHT. Furthermore, we have measured the gas-phase infrared and liquid-phase Raman spectra of PHT and BZD and performed the normal mode analysis at the

B3LYP/6-31G(d) level. The fundamentals of those molecules have been assigned by comparing the experimental band positions and intensities with the calculated ones. This work will provide a better theoretical basis for understanding the conformational features of oxygen-containing benzene-fused ring molecules such as PHT and BZD. 2. Calculations The molecular orbital energy calculations were performed using the gaussian 98 program package [13]. The geometries of the Cs and C2v conformers and the energy barrier to planarity for PHT were calculated at the HF, B3LYP and MP2 levels with the 631G(d) basis set. The single point energy calculations with larger basis sets and with MP4 correlation treatments were also performed to predict a reliable barrier height. NBO analysis at the B3LYP/6-311G(d,p) level was carried out to understand some of the factors contributing to the total conformational energy. With the NBO deletion procedure, the energy of the hyperconjugative interaction of interest was evaluated by calculating the change in the sum of the energies of occupied orbitals upon deletion of speci®c offdiagonal elements of the Fock matrix in the NBO basis [14±16]. In this way, the total energy was decomposed into components associated with covalent and non-covalent contributions, and therefore the delocalization effects were estimated from comparison of the deletion energies. The infrared and Raman frequencies and band intensities for BZD and PHT were calculated at the B3LYP level of theory with the 6-31G(d) basis set. This is because most of recent publications indicate that the B3LYP functional is the preferred method for the vibrational analysis. Frequency calculations at this level show that optimized structures of all molecules are at stationary points corresponding to local minima without imaginary frequencies. The calculated infrared and Raman spectra of BZD and PHT were ®t using Lorentzian band shapes in order to compare with the observed frequencies. The gaussview program was also used to visualize the vibrational modes to assist in the assignment of spectra.

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161

Table 1 Calculated ring-puckering barrier heights of phthalan using several basis sets (energies in cm 21) Basis set

Basis functions

HF a

B3LYP b

MP2 c

MP3 c

MP4 c

6-31G(d) 6-31G(d,p) 6-311G(d) 6-311G(d,p)

151 171 186 210

0 0 0 0

12 11 10 10

92 75 145 117

26 10 68 42

88 70 140 111

a b c

Single point energy calculations performed at the respective HF(full)/6-31G(d) geometry. Single point energy calculations performed at the respective B3LYP(full)/6-31G(d) geometry. Single point energy calculations performed at the respective MP2(full)/6-31G(d) geometry.

3. Experimental The gas-phase mid-infrared spectra of PHT and BZD were recorded on a Bio-Rad FTS-6000 interferometer equipped with a Cryogenic MCT detector. Gas-phase spectra were recorded at a resolution of 0.5 cm 21 at 5 Torr of pressure in a 4 m multi-re¯ectance cell with KBr windows, while liquid-phase farinfrared spectra were recorded at a resolution of 1 cm 21 using a DTGS detector and a 2.0 mm wavelength polyethylene cell. Liquid-phase Raman spectra were recorded on a Bruker IFS-55 FT-Raman spectrophotometer using Nd:Yag laser as an excitation source. A liquid nitrogen cooled Germanium detector and CaF2 beam splitter were used for the FT-Raman measurements. The laser line at 1064 nm was used with 500 mW of power. Typically, 500 scans at 1 cm 21 resolution were employed. PHT and BZD were purchased from Aldrich Chemical and used without further puri®cation.

4. Results and discussion 4.1. Barrier to planarity of phthalan Recently, Sakurai et al. [6] reported the two-dimensional vibrational potential energy surface of PHT from the analysis of far-infrared spectra. This surface does an excellent job of reproducing the irregular pattern of observed energy spacings for the ring-puckering vibration. The potential energy surface has a barrier to planarity of 35 cm 21. In order to get a theoretical puckering barrier of PHT, the energy difference between the planer (C2v) and puckered (Cs) conformers has been calculated at various levels

of theory. The single-point energy calculations with basis sets much larger than 6-31G(d) and with postMP2 correlation treatments have also been performed. The results are summarized in Table 1. In the case of the HF method, the calculation predicts that PHT has a planar equilibrium structure. On the other hand, both the B3LYP and MP results, including electron correlation, indicate that PHT has a tiny ring-puckering barrier. The very high level calculation at the QCISD(T)/6-31G(d) predicts the barrier of 72 cm 21. Even though we cannot estimate the barrier height at the basis set limit due to a limited computer CPU time, it is clear that PHT has a tiny ring-puckering barrier. This appears to be in reasonable agreement with the experimental result. It has long been known that the relative balance between two opposing forces determines the equilibrium conformation ring molecule. Ring angle strain tends to keep the ring planar, whereas torsional strain, caused by CH2 ±CH2 interactions, tends to pucker the ring. In this point of view, PHT is expected to be planar since it has no adjacent CH2 group. However, both the spectroscopic and computational results show that PHT has a tiny puckering barrier. Thus, those two opposing steric forces cannot successfully explain the unexpected non-planarity of PHT. In order to better explain the physical origin of tiny puckering barrier for PHT, the NBO analysis around the C±O bonds of ®ve-membered ring has been performed. 4.2. NBO analysis of phthalan As shown in Table 1, the B3LYP calculation slightly underestimates the barrier to planarity

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Table 2 Energy contributions (kcal mol 21) of total delocalization (D1 del) and Lewis energies (D1 Lewis) to the energy differences (DEtot) between the C2v and Cs conformers of indan and phthalan D(C2v 2 Cs) a

Indan Phthalan a b

Table 3 Analysis of the hyperconjugatve contributions (kcal mol 21) to the main orbital interaction energies of C2v and Cs conformers of indan and phthalan Donor

D1 Lewis

2 D1 del b

DEtot

2 0.58 2 0.42

1.65 0.45

1.07 0.03

Energy difference between the planar and puckered forms. D1 del ˆ D1 Lewis 2 DEtot.

compared to the experimental or MP results. The B3LYP method, however, predicts a qualitatively correct double-minimum potential energy pro®le even though the role played by electron correlation at the MP2 level is very important to determine the accurate barrier height. Thus, the NBO analysis of B3LYP wavefunctions is able to properly identify the main driving forces, even though the myriad modulating forces are not precisely balanced at this level, leading to underestimations of the barrier. It was reported that indan has much higher

C2v a

Cs b

D(C2v 2 Cs)

Indan: main delocalizations from the s (C8 ±C15) orbital s(C8 ±C15) s p(C11 ±H16) 0.58 0.03 0.55 s p(C11 ±H17) 0.58 1.59 2 1.01 Sum 1.16 1.80 2 0.46 Phthalan: main delocalizations from the s (C8 ±O15) orbital s(C8 ±O15) s p(C11 ±H16) 0.28 0.12 0.16 s p(C11 ±H17) 0.28 0.49 2 0.21 Sum 0.56 0.61 2 0.05 Indan: main delocalizations from the s (C15 ±H18) and s (C15 ±H19) orbitals s(C15 ±H18) s p(C4 ±C8) 0.76 1.63 2 0.87 s p(C8 ±H13) 0.71 0.74 2 0.03 s p(C8 ±H14) 1.03 0.02 1.01 s p(C4 ±C8) 0.76 0.06 0.70 s(C15 ±H19) s p(C8 ±H13) 1.03 2.21 2 1.18 s p(C8 ±H14) 0.71 0.74 2 0.03 Sum 5.00 5.40 2 0.40 Phthalan: main delocalizations from the n1(O15) orbital n1(O15) s p(C4 ±C8) 2.86 2.71 0.15 0.66 0.22 0.44 s p(C8 ±H13) s p(C8 ±H14) 0.66 1.21 2 0.55 n2(O15) s p(C4 ±C8) 0.00 0.32 2 0.32 5.55 7.10 2 1.55 s p(C8 ±H13) s p(C8 ±H14) 5.55 3.79 1.80 Sum 15.28 15.35 2 0.07 a b

Fig. 1. Numbering scheme of: (a) indan and (b) phthalan.

Acceptor

Planar conformation. Puckered conformation.

puckering barrier than PHT [7]. If we analyze and compare the orbital interactions to pucker the rings for indan and PHT by the NBO deletion procedure, it might be possible to understand the main driving forces for the tiny puckering barrier of PHT. For this purpose, the energy contributions of the delocalization energies (D1 del) and Lewis energies (D1 Lewis: all energy contributions apart from delocalization effects) have been calculated for indan and PHT, respectively. The results are listed in Table 2. As shown in the table, D1 del is likely to contribute to the stabilization of the puckered form of both the indan and PHT. However, the contribution of the delocalization energy to the total energy is much larger in the case of indan. As a result, indan has a higher puckering barrier than phthalan.

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163

Table 4 Observed and calculated vibrational frequencies (cm 21) of phthalan (abbreviations: s, strong; m, medium; w, weak; v, very; numbers in brackets are infrared frequencies observed from pure-liquid far-infrared spectra) Vibrational descriptions (C2v)

Observed Infrared (vapor)

Raman (liquid)

Literature

B3LYP/6-31G(d) a

A1 n1 n2 n3 n4 n5 n6 n7 n8 n9 n10 n11 n12 n13 n14 n15

CH sym. stretch CH sym. stretch CH2 sym. stretch Benzene ring stretch CH2 deformation Benzene ring stretch CH2 wag Benzene ring stretch CH in-plane wag CH in-plane wag Benzene ring stretch Ring stretch Benzene ring bend Benzene ring bend Ring bend

3089 s 3045 s 2925 s 1594 m 1530 vw 1480 s 1371 s 1265 m 1168 w ± 1021 m 915 s 808 m 694 m ±

3079 vs 3051 vs 2899 s 1591 s 1474 s ± 1371 s 1293 w 1224 s ± 1023 vs 902 s 817 s 696 s 517 vs

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

3085 (24.4/248) b 3062 (2.3/138) 2877 (23.6/333) 1587 (0.7/9.6) 1495 (0.0/32.4) 1469 (10.6/1.6) 1364 (26.2/19.7) 1297 (0.9/6.0) 1205 (0.1/13.3) 1146 (0.4/5.6) 1014 (1.5/16.0) 897 (19.4/3.7) 811 (7.3/17.6) 686 (4.5/3.5) 502 (0.0/10.8)

A2 n16 n17 n18 n19 n20 n21 n22 n23

CH2 antisym. stretch CH2 twist CH2 rock CH out-of-plane wag CH out-of-plane wag Benzene ring twist Benzene out-of-plane bend Ring twist

± ± ± ± ± ± ± ±

2936 s 1182 m 1042 w ± 862 vw ± ± 204 s

± ± ± ± ± ± 543 c 226 c, 186 d

2925 (0.0/179) 1180 (0.0/5.8) 1029 (0.0/0.3) 945 (0.0/0.1) 847 (0.0/4.6) 682 (0.0/0.2) 482 (0.0/0.2) 177 (0.0/1.0)

B1 n24 n25 n26 n27 n28 n29 n30

CH2 antisym. stretch CH2 twist CH2 rock CH out-of-plane wag CH out-of-plane wag Benzene out-of-plane bend Ring ¯apping

n31

Ring puckering

2964 s 1119 w 989 w 943 w 738 vs [412] w [234] w ± ±

2952 s 1153 m 996 vw ± 745 vw 414 w 233 s ± 92 e vs

± ± ± ± ± 407 c 219 c, 216 d 67 c, 31 d 105 e

2929 (102/77.3) 1144 (0.0/14.4) 993 (0.5/0.1) 900 (0.8/1.5) 730 (49.0/1.3) 408 (1.7/0.2) 216 (2.4/5.3) 67 (7.4/2.4) ±

B2 n32 n33 n34 n35 n36 n37 n38 n39 n40 n41 n42 n43 n44 n45

CH antisym. stretch CH antisym. stretch CH2 sym. stretch Benzene ring stretch CH2 deformation Benzene ring stretch CH2 wag CH in-plane wag CH in-plane wag Ring stretch Ring stretch Benzene ring bend Benzene ring bend Ring bend

3068 s 3037 s 2877 vs 1607 m 1503 vw 1460 m 1327 w 1236 w ± 1096 m 1058 vs 829 m ± [366] vw

± 3015 s 2857 vs 1612 s 1467 s ± 1327 w 1264 m ± 1118 vw 1099 w 833 s 593 m 374 w

± ± ± ± ± ± ± ± ± ± ± ± ± 370 c

3073 (32.1/53.2) 3057 (5.0/22.6) 2871 (124/82.3) 1605 (1.2/16.3) 1482 (3.6/30.2) 1450 (7.3/0.1) 1313 (0.0/6.4) 1247 (3.6/8.5) 1156 (0.1/0.6) 1087 (7.3/1.7) 1055 (91.7/0.6) 826 (0.0/0.0) 581 (0.0/5.2) 363 (0.3/0.5)

a b c d e

Calculated vibrational frequencies scaled by 0.9613. Calculated infrared/Raman intensities. Ref. [1]. Ref. [17]. Double quantum jump of ring puckering mode.

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Table 5 Observed and calculated vibrational frequencies (cm 21) of 1,3-benzodioxole (abbreviations: s, strong; m, medium; w, weak; v, very; numbers in brackets are infrared frequencies observed from pure-liquid far-infrared spectra) Vibrational descriptions (C2v)

Observed Infrared (vapor)

Raman (liquid)

Literature

B3LYP/6-31G(d) a

A1 n1 n2 n3 n4 n5 n6 n7 n8 n9 n10 n11 n12 n13 n14

CH sym. stretch CH sym. stretch CH2 sym. stretch Benzene ring stretch CH2 deformation Benzene ring stretch Benzene ring stretch Benzene ring stretch CH in-plane wag Ring stretch CH in-plane wag Benzene ring bend Benzene ring bend Ring bend

3112 w 3075 s 2884 s 1624 w 1514 s 1487 vs 1365 s 1240 vs 1151 w 1053 vs 998 vw 799 m 725 m [536] vw

3103 s 3063 vs 2892 vs 1628 s 1501 s 1479 w 1362 s 1232 s 1140 vw 1039 m 1003 vs 798 vs 713 vs 537 s

± ± ± ± ± ± ± ± ± ± ± 799 c 736 c 534 c

3104 (2.1/260) b 3085 (18.9/124) 2907 (132/218) 1618 (1.7/5.3) 1514 (0.5/25.4) 1474 (258/0.8) 1360 (24.3/4.6) 1238 (283/7.4) 1142 (2.7/4.8) 1033 (106/0.5) 995 (1.2/14.3) 789 (9.6/20.6) 704 (2.8/10.8) 527 (1.5/9.4)

A2 n15 n16 n17 n18 n19 n20

CH2 twist CH out-of-plane wag CH out-of-plane wag Benzene ring twist Benzene out-of-plane bend Ring twist

± ± ± ± ± ±

1152 s ± 825 w 646 vw 561 vw 225 s

± ± ± ± 614 c 214 c

1168 (0.0/10.7) 918 (0.0/0.0) 822 (0.0/4.8) 686 (0.0/0.3) 545 (0.0/0.1) 211 (0.0/3.5)

B1 n21 n22 n23 n24 n25 n26 n27

CH2 antisym. stretch CH2 rock CH out-of-plane wag CH out-of-plane wag Benzene out-of-plane bend Ring ¯apping Ring puckering

2929 m 1097 m 908 w 736 vs [419] vw [254] vw ±

2983 s 1093 w ± 740 vw 420 w 285 s 89 vs

± ± ± ± 405 c, 413 d 267 c, 236 d 91 c, 158 d

2992 (57.5/152) 1110 (10.9/1.3) 876 (5.0/2.1) 723 (62.8/1.3) 410 (1.1/0.3) 269 (0.0/4.3) 79 (9.0/0.2)

B2 n28 n29 n30 n31 n32 n33 n34 n35 n36 n37 n38 n39

CH antisym. stretch CH antisym. stretch Benzene ring stretch Benzene ring stretch CH2 wag CH in-plane wag Ring stretch CH in-plane wag Ring stretch Benzene ring bend Benzene ring bend Ring bend

3081 s 3030 m 1580 w 1461 vw 1401 w 1261 m 1123 w 1087 m 954 s 854 m ± [412] w

3078 vs 3033 s 1605 s 1464 s 1401 m 1280 w 1125 vw ± ± 841 m 612 s ±

± ± ± ± ± ± ± ± ± ± ± ±

3101 (12.8/8.8) 3071 (2.1/60.7) 1599 (4.2/11.2) 1452 (2.1/7.8) 1394 (3.0/15.6) 1264 (0.8/1.0) 1129 (2.7/4.6) 1077 (14.9/0.8) 938 (44.5/0.0) 830 (14.9/1.4) 601 (0.0/4.8) 398 (3.3/0.1)

a b c d

Calculated vibrational frequencies scaled by 0.9613. Calculated infrared/Raman intensities. Ref. [12]. Ref. [1].

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165

Fig. 2. Observed and calculated infrared spectra of phthalan: (a) gas-phase mid-infrared spectrum; (b) liquid-phase far-infrared spectrum; and (c) calculated infrared spectrum at the B3LYP/6-31G(d) level.

Fig. 4. Observed and calculated infrared spectra of 1,3-benzodioxole: (a) gas-phase mid-infrared spectrum; (b) liquid-phase farinfrared spectrum; and (c) calculated infrared spectrum at the B3LYP/6-31G(d) level.

The numbering schemes of atoms for indan and PHT are depicted in Fig. 1. To better understand the origin of the main orbital interactions for the contribution of delocalization energy to total energy, we have decomposed D1 del into several components around the C8 ±C15 for indan and the C8 ±O15 for PHT, respectively. Nine hyperconjugative energy contributions to the energy differences between C2v and Cs structures for indan and PHT are summarized in Table 3. It is noteworthy that the electron delocalizations from the s(C8 ±C15) orbital and the s(C8 ±O15) orbital stabilize the puckered conformations of the molecules. In the case of indan, the electron delocalization from the s(C8 ±C15) orbital in the puckered form is much more favorable than that in the planar

form. The orbital interaction energies for the s(C8 ± C15) ! s p(C11 ±H16) and s(C8 ±C15) ! s p(C11 ±H17) are same as 0.58 kcal mol 21 in the planar structure, but the energy (0.03 kcal mol 21) for the s(C8 ± C15) ! s p(C11 ±H16) is much weaker than the energy (1.59 kcal mol 21) for the s(C8 ±C15) ! s p(C11 ±H17) in the puckered form. Due to those orbital interaction energy differences, the puckered form is more stabilized. Similar trend is also observed for phthalan but the amount of stabilization is weaker than that of indan. The electron delocalizations, from s(C±H) orbital of indan and from the oxygen non-bonding orbital of PHT, have also been considered. In the case of indan, the total energy difference between the planar and puckered forms is 20.40 kcal mol 21. This means that the puckered form is greatly stabilized compared to the planar form. The energy stabilization of puckered form, caused by the oxygen non-bonding orbital of PHT, also shows a similar trend but it is relatively weaker than that of indan. The NBO analysis for the other molecular orbital interactions has also been considered but the contributions to non-planar conformational stabilization are relatively small compared to the molecular orbital delocalization around the C± O bonds of ®ve-membered ring. The NBO analysis at the B3LYP/6-311G(d,p) level indicates that the tiny puckering barrier of PHT is caused by the co-operative contributions of electron delocalizations from the s(C±O) and oxygen non-bonding orbitals of ®vemembered ring.

Fig. 3. Observed and calculated Raman spectra of phthalan: (a) liquid-phase FT-Raman spectrum; and (b) calculated Raman spectrum at the B3LYP/6-31G(d) level.

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Fig. 5. Observed and calculated Raman spectra of 1,3-benzodioxole: (a) liquid-phase FT-Raman spectrum; and (b) calculated Raman spectrum at the B3LYP/6-31G(d) level.

4.3. Vibrational analysis of phthalan and 1,3benzodioxole Normal vibrational mode analyses of PHT and BZD have been performed in terms of C2v symmetry. A comparison of experimental and B3LYP theoretical infrared and Raman spectra have been listed in Tables 4 and 5. The previously reported vibrational frequencies [1,12,17] are also shown in the tables. The simulated infrared and Raman spectra using Lorentzian band shapes are compared to the observed spectra in Figs. 2±5. For a better comparison with the observed frequencies, the frequencies calculated at the B3LYP/ 6-31G(d) level have been uniformly scaled down by a factor of 0.9613 [18]. As shown in Figs. 2 and 4, the frequencies and intensities of the experimental infrared spectra of PHT and BZD are quite nicely reproduced by the B3LYP calculations showing almost a one-to-one correspondence. The calculated Raman frequencies are also satisfactory. However, it is indicated in Figs. 3 and 5 that the B3LYP Raman intensities are not consistent with the experimental ones in the whole spectral range.

5. Conclusions In the present study, the ring-puckering conformation and the origin of tiny puckering barrier on PHT have been examined by the ab initio calculation and NBO analysis. The calculated ringpuckering barrier height with high level electron

correlation treatments with extended basis set appears to be in reasonable agreement with the experiment. The NBO analysis at the B3LYP/6311G(d,p) level indicates that the tiny ring-puckering barrier is caused by a wide variety of electron delocalizations. However, the co-operative contributions of electron delocalizations from the s(C±O) and oxygen non-bonding orbitals of the ®ve-membered ring are the main driving forces to pucker the ring. We have also shown the accuracy of DFT method using the B3LYP hybrid functional in predicting the vibrational frequencies and absorption intensities for PHT and BZD. Acknowledgement This work is supported by the Korea Science and Engineering Foundation (Grant no. R-02-200100172). References [1] T.L. Smithson, J.A. Duckett, F.L. Bettens, J. Phys. Chem. 88 (1984) 1102. [2] K.H. Hassan, J.M. Hollas, J. Mol. Spectrosc. 147 (1991) 100. [3] K.H. Hassan, J.M. Hollas, Chem. Phys. Lett. 169 (1990) 17. [4] W. Caminati, D. Damiani, L.B. Favero, Mol. Phys. 79 (1993) 699. [5] S. Sakurai, N. Meinander, J. Laane, J. Chem. Phys. 108 (1998) 3531. [6] S. Sakurai, N. Meinander, J. Laane, J. Chem. Phys. 108 (1998) 3537. [7] J. Laane, J. Phys. Chem. A 104 (2000) 7715. [8] E. Bondoc, S. Sakurai, K. Morris, W.-Y. Chiang, J. Laane, J. Chem. Phys. 112 (2000) 6700. [9] S. Moon, Y. Kwon, J. Lee, J. Choo, J. Phys. Chem. A 105 (2001) 3221. [10] J. Choo, J. Mol. Struct. 587 (2001) 235. [11] S. Sakurai, N. Meinander, K. Morris, J. Laane, J. Am. Chem. Soc. 121 (1999) 50. [12] J. Laane, E. Bondoc, S. Sakurai, K. Morris, N. Meinander, J. Choo, J. Am. Chem. Soc. 122 (2000) 2628. [13] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheesman, V.G. Zakrzewski, J.A. Montgomery, R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clliford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowsski, J.V. Ortiz, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham,

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