Transition-state variation in the nucleophilic substitution reactions of aryl bis(4-methoxyphenyl) phosphates with pyridines in acetonitrile

Article abstract of DOI:10.1021/jo0162742

The kinetics and mechanism of the reactions of Z-aryl bis(4-methoxyphenyl) phosphates, (4-MeOC₆H₄O)₂P(=O) OC₆H₄Z, with pyridines (XC₅H₄N) are investigated in acetonitrile at 55.0 °C. In the case of more basic phenolate leaving groups (Z = 4-Cl, 3-CN), the magnitudes of β_x (β_nuc) and β_z (β_lg) indicate that mechanism changes from a concerted process (β_x = 0.22-0.36, β_z = -0.42 to -0.56) for the weakly basic pyridines (X = 3-Cl, 4-CN) to a stepwise process with rate-limiting formation of a trigonal bipyramidal pentacoordinate (TBP-5C) intermediate (β_x = 0.09-0.14, β_z = -0.08 to -0.28) for the more basic pyridines (X = 4-NH₂, 3-CH₃). This proposal is supported by a large negative cross-interaction constant (ρ_xz = -1.98) for the former and a positive ρ_xz(+0.97) for the latter processes. In the case of less basic phenolate leaving groups (Z = 3-CN, 4-NO₂), the unusually small magnitude of β_z values is indicative of a direct backside attack TBP-5C TS in which the two apical sites are occupied by the nucleophile and leaving group, ap(NX)-ap(LZ). The instability of the putative TBP-5C intermediate leading to a concerted displacement is considered to result from relatively strong proximate charge transfer interactions between the π-lone pairs on the directly bonded equatorial oxygen atoms and the apical bond (n_o(eq) - ?*(ap)). These are supported by the results of natural bond orbital (NBO) analyses at the NBO-HF/6-311+G**//B3LYP/6-311+G** level of theory.

Full text of DOI:10.1021/jo0162742

J . Org. Chem. 2002, 67, 2215-2222
2215
Tr a n sition -Sta te Va r ia tion in th e Nu cleop h ilic Su bstitu tion
Rea ction s of Ar yl Bis(4-m eth oxyp h en yl) P h osp h a tes w ith
P yr id in es in Aceton itr ile
Hai Whang Lee,* Arun Kanti Guha,† Chang Kon Kim, and Ikchoon Lee*
Department of Chemistry, Inha University, Inchon 402-751, Korea
hwlee@inha.ac.kr
Received November 7, 2001
The kinetics and mechanism of the reactions of Z-aryl bis(4-methoxyphenyl) phosphates, (4-
MeOC6H4O)2P(dO)OC6H4Z, with pyridines (XC5H4N) are investigated in acetonitrile at 55.0 °C. In
the case of more basic phenolate leaving groups (Z ) 4-Cl, 3-CN), the magnitudes of âX (ânuc) and
âZ (âlg) indicate that mechanism changes from a concerted process (âX ) 0.22-0.36, âZ ) -0.42 to
-0.56) for the weakly basic pyridines (X ) 3-Cl, 4-CN) to a stepwise process with rate-limiting
formation of a trigonal bipyramidal pentacoordinate (TBP-5C) intermediate (âX ) 0.09-0.14, âZ )
-0.08 to -0.28) for the more basic pyridines (X ) 4-NH2, 3-CH3). This proposal is supported by a
large negative cross-interaction constant (FXZ ) -1.98) for the former and a positive FXZ (+0.97) for
the latter processes. In the case of less basic phenolate leaving groups (Z ) 3-CN, 4-NO2), the
unusually small magnitude of âZ values is indicative of a direct backside attack TBP-5C TS in
which the two apical sites are occupied by the nucleophile and leaving group, ap(NX)sap(LZ). The
instability of the putative TBP-5C intermediate leading to a concerted displacement is considered
to result from relatively strong proximate charge transfer interactions between the π-lone pairs on
the directly bonded equatorial oxygen atoms and the apical bond (nO(eq) - σ*(ap)). These are
supported by the results of natural bond orbital (NBO) analyses at the NBO-HF/6-311+G**//B3LYP/
6-311+G** level of theory.
In tr od u ction
The great interest in phosphorus chemistry stems from
its relevance to biological chemistry and from its useful-
ness in synthesis. Two main types of displacement
process are known in neutral phosphoryl group transfer
reaction.1 The stepwise mechanism involves a trigonal
bipyramidal pentacoordinate (TBP-5C) intermediate and
the concerted displacement at phosphorus through a
single pentacoordinate transition state (TS).1,2 In the
former, the addition step can precede the elimination
(addition-elimination) or it can be reversed to elimina-
tion-addition.1 There is a further complication involving
stereochemical results since inversion or retention of
configuration can be obtained depending on whether the
attacking nucleophile (NX) and leaving group (LZ) occupy
the apical positions (I: ap(NX)sap(LZ)) or an apical and
an equatorial positions (ap(NX)seq(LZ)) in the TBP-5C
adduct. In the latter case, a Berry type pseudorotation
(BPR)3 leads subsequently to an eq(NX)sap(LZ) adduct,
eq 1. In the concerted retention process, a front-side
nucleophilic attack occurs, II.
The change in mechanism from a concerted to a
stepwise reaction has been shown to occur by varying the
strength of the nucleophile and leaving group in the
neutral phosphoryl group transfer.4 The concerted path
becomes more likely to be followed with less basic (or
weaker) nucleophiles and with stronger apicophilic (or
nucleofugal) leaving group. For very basic (or strong)
nucleophiles and poor (less necleofugal) leaving groups
(e.g., PhO-), a stepwise path is favored with an ap(NX)s
eq(LZ) type TBP-5C adduct that pseudorotates before loss
of the leaving group to an eq(NX)sap(LZ) form.4
* To whom correspondence should be addressed. Fax: +82-32-865-
4855.
† Present address: Independent University, Dhaka, Bangladesh.
(1) Williams, A. Concerted Organic and Bio-organic Mechanisms;
CRC Press: Boca Raton, 2000; Chapter 6.
(2) (a) Hudson, R. F. Structure and Mechanism in Organophospho-
rus Chemistry; Academic Press: New York, 1965. (b) Lee, I.; Kim, C.
K.; Li, H. G.; Sohn, C. K.; Kim, C. K.; Lee, H. W.; Lee, B.-S. J . Am.
Chem. Soc. 2000, 122, 11162.
In this work, we examined mechanistic changes in-
volved in the nucleophilic substitution of aryl bis(4-
(3) Berry, R. S. J . Chem. Phys. 1960, 32, 933.
(4) Rowell, R.; Gorenstein, D. G. J . Am. Chem. Soc. 1981, 103, 5894.
10.1021/jo0162742 CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/13/2002

2216 J . Org. Chem., Vol. 67, No. 7, 2002
Lee et al.
Ta ble 1. Secon d -Or d er Ra te Con sta n ts (k2 × 104/M-1 s-1
for th e Rea ction s of Z-Ar yl Bis(4-m eth oxyp h en yl)
P h osp h a tes w ith X-P yr id in es in Aceton itr ile a t 55.0 °C
)
Ta ble 2. Selectivity P a r a m eter sa GX, âX, GZ, âZ, a n d GXZ for
th e Rea ction s of Z-Ar yl Bis(4-m eth oxyp h en yl)
P h osp h a tes w ith X-P yr id in es in Aceton itr ile a t 55.0 °C
Z
(Z ) 4-Cl, 3-CN)
FZ âZ
(Z ) 3-CN, 4-NO2)
FZ âZ
X
X
4-Cl
67.0
24.0
21.4
20.0
3-Cl
75.1
34.7
29.0
27.2
3-CN
4-CN
4-NO2
4-NH2
4-CH3
0.30 ( 0.02b -0.08 ( 0.01 0.13 ( 0.03 -0.01 ( 0.001
0.87 ( 0.13 -0.21 ( 0.06 0.09 ( 0.05 -0.01 ( 0.005
4-C6H5CH2 0.85 ( 0.04 -0.28 ( 0.02 0.22 ( 0.03 -0.02 ( 0.001
4-NH2
4-CH3
4-C6H5CH2
3-CH3
H
85.1
46.8
40.7
38.0
8.12
2.31
2.03
1.21
1.03
0.679
88.3
46.6
43.7
42.8
8.55
2.63
2.14
1.26
1.07
0.708
91.2
49.2
45.7
44.7
9.03
2.88
2.57
1.41
1.12
0.776
3-CH3
0.84 ( 0.06 -0.21 ( 0.04 0.30 ( 0.08 -0.02 ( 0.004
XZ ) 0.97 ( 0.15 FXZ ) 0.18 ( 0.14
2.25 ( 0.12 -0.56 ( 0.04 0.43 ( 0.06 -0.03 ( 0.01
F
7.17
7.58
3-Cl
3-C1
0.418
0.392
0.273
0.248
0.198
0.790
0.776
0.423
0.361
0.268
3-CH3CO 2.18 ( 0.02 -0.53 ( 0.07 0.44 ( 0.13 -0.03 ( 0.01
4-CH3CO 1.96 ( 0.28 -0.50 ( 0.01 0.31 ( 0.07 -0.02 ( 0.01
3-CH3CO
4-CH3CO
3-CN
3-CN
4-CN
1.91 ( 0.32 -0.47 ( 0.02 0.17 ( 0.01 -0.01 ( 0.001
1.63 ( 0.33 -0.42 ( 0.03 0.26 ( 0.04 -0.02 ( 0.01
4-CN
F
XZ ) -1.98 ( 0.77
FXZ ) -0.81 ( 0.7
methoxyphenyl) phosphates (1), with pyridines in ace-
tonitrile at 55.0 °C, eq 2, by determining the Hammett
(X ) 4-NH2 ∼ 3-CH3)
Z
FX
âX
4-Cl
3-Cl
3-CN
4-CN
4-NO2
-0.89 ( 0.02
-0.74 ( 0.04
-0.57 ( 0.03
-0.54 ( 0.01
-0.53 ( 0.02
0.14 ( 0.003
0.12 ( 0.008
0.09 ( 0.006
0.09 ( 0.02
0.09 ( 0.02
(X ) 3-Cl ∼ 4-CN)
Z
FX
âX
4-Cl
3-Cl
3-CN
4-CN
4-NO2
-1.11 ( 0.07
-1.67 ( 0.13
-1.77 ( 0.09
-1.86 ( 0.15
-1.96 ( 0.11
0.22 ( 0.02
0.35 ( 0.01
0.36 ( 0.05
0.36 ( 0.05
0.39 ( 0.05
a
σ values were taken from: Hansch, C.; Leo, A.; Taft, R. W.
Chem. Rev. 1991, 91, 165. The pKa values of pyridines in water at
25 °C were taken from ref 7 and: Dean, J . A. Handbook of Organic
Chemistry; McGraw-Hill: New York, 1987; Chapter 8. The pKa
values of phenols in CH3CN at 25 °C were taken from ref 8, and
(FX, FZ) and Bro¨nsted (âX, âZ) type selectivity parameters
including the cross-interaction constants,5
FXZ, eqs 3,
b
the pKa value of 3-cyanophenol was determined by eq 5b. Stan-
dard deviations.
where X and Z represent substituents in the nucleophile
(NX) and leaving group (LZ), respectively. To substanti-
ate our arguments theoretically, we have carried out
density functional theory (DFT) calculations on various
TBP-5C adduct structures.
âZ) derived from the linear portion of these plots and
Hammett coefficients (FX, FZ) are summarized in Table
2. It is noted that all the plots, Figures 1 and 2, are
biphasic with break points at X ) H and Z ) 3-CN,
respectively, in the variation of the nucleophile and
leaving group. For the more basic (stronger) nucleophiles
(X ) 4-NH2, 3-CH3), the magnitudes of FX () -0.53 to
-0.89) and âX () 0.09-0.14) are smaller than those for
the less basic (weaker) nucleophiles (X) 3-Cl, 4-CN)
(FX ) -1.11 to -1.96 and âX ) 0.22-0.39). Similarly, for
the former (stronger nucleophiles), the magnitudes of FZ
and âZ are smaller than those for the latter less basic
(weaker) nucleophiles. On the other hand, the magni-
tudes of FZ and âZ are also in two distinct groups with
larger values for more basic (poorer) leaving groups
(Z ) 4-Cl, 3-CN) and smaller ones for less basic (better)
leaving groups (Z ) 3-CN, 4-NO2).
log(kXZ/kHH) ) FXσX + FZσZ + FXZσXσZ
∂FZ ∂FX
(3a)
(3b)
FXZ
)
)
∂σX ∂σZ
Resu lts a n d Discu ssion
The reactions were carried out under pseudo-first-order
conditions with a large excess of pyridine. The pseudo-
first-order rate constants observed (kobs) for all the
reactions followed eq 4
kobs ) k0 + k2[Py]
(4)
with negligible k0 () 0) in acetonitrile. The second-order
rate constants, k2 (M-1 s-1), summarized in Table 1 were
determined using eq 4 with at least five pyridine con-
centrations, [Py]. No third-order or higher-order terms
were detected, and no complications arising from side
reaction were found in the determination of kobs and in
the linear plots of eq 4. This suggests that the overall
reaction follows cleanly the route given by eq 2.
It should be noted that in the determination of the
Bro¨nsted coefficients, âX, we correlated log k2 (MeCN)
with pKa(H2O) of pyridines, which has been shown to be
justified.6 For the determination of âZ values, however,
we have tried correlations of log k2(MeCN) with both the
(6) (a) Lee, I.; Kim, C. K.; Han, I. S.; Lee, H. W.; Kim, W. K.; Kim,
Y. B. J . Phys. Chem. B 1999, 103, 7302. (b) Coetzee, J . F. Prog. Phys.
Org. Chem. 1965, 4, 45. (c) Spillane, W. J .; Hogan, G.; McGrath, P.;
King, J .; Brack, C. J . Chem. Soc., Perkin Trans. 2 1996, 2099. (d)
Foroughifar, N.; Leffek, K. T.; Lee, Y. G. Can. J . Chem. 1992, 70, 2856.
(e) Ritchie, C. D. Solute-Solvent Interactions; Marcel-Dekker: New
York, 1969; p 228. (f) Koh, H. J .; Han, K. L.; Lee, H. W.; Lee, I. J . Org.
Chem. 1998, 63, 9834.
The rate constants k2 are correlated by the Bro¨nsted
plots in Figures 1 and 2. The Bro¨nsted coefficients (âX,
(5) (a) Lee, I. Adv. Phys. Org Chem. 1992, 27, 57. (b) Lee, I. Chem.
Soc. Rev. 1994, 24, 223.

Substitution Reactions of Aryl Bis(4-methoxyphenyl) Phosphates
J . Org. Chem., Vol. 67, No. 7, 2002 2217
pKa(H2O)7 and pKa(MeCN)8 values of phenols. Available
pKa values gave the following relations, eqs 5, for phenols.
These relations indicate that the Bro¨nsted basicities of
phenolates change by ca. twice in acetonitrile relative to
those in water (δpKa(MeCN)/δpKa(H2O) = 2.1) for a given
substituent changes (δσ-). In contrast, however, the ratio
of âZ, i.e., changes in rates (δ log k2) (Lewis basicities)
with pKa’s for a given substituent changes (δpKa) are only
an average of 1.6 () âZ with pKa(H2O)/âZ with pKa-
(MeCN)); i.e., the ratios differ by a factor of ca. 0.76
() 1.6/2.1). This is of course due to the partial bond
cleavage (or formation) of phenolate anions in the rate-
determining step (in the TS) in contrast to the complete
deprotonation (or protonation) in the pKa measurements
of pyridines.
for concerted processes have been reported in phosphoryl
group transfer reactions.14 Skoog and J encks14a reported
âX ) 0.17 and 0.19 for the concerted reactions of pyridines
with phosphorylated 3-methoxypyridine and âX ) 0.22
and 0.28 for the reaction with phosphorylated 4-mor-
pholinopyridine, respectively. Williams and co-workers14b
obtained âX ) 0.15 for the similar concerted reaction.
Williams and co-workers14d reported also the similar
magnitudes of âX () 0.14-0.60) and âZ () -0.52 to -0.81)
values for the concerted reactions of phenolate anions
with phenyl diphenyl phosphate ((PhO)2P(dO)OAr +
ArO-). These are certainly similar to or even smaller than
the âX values obtained for the weakly basic nucleophiles
in the present work. It should be noted, however, that
âX values larger than 0.7 have been also found in other
concerted reactions.15 On the other hand, the magnitudes
of âX and âZ values in a stepwise mechanism with the
rate-limiting bond formation of T( are in general sub-
stantially smaller (âX ) 0.0-0.3 and âZ ) -0.1 to -0.3)16
than those for the concerted reactions. For example, âX
values in the range âX ) 0.19-0.23 have been obtained
for the stepwise reactions with rate-limiting bond making
in the pyridinolysis of S-phenyl 4-nitrobenzoates in
acetonitrile, XC5H4N + 4-NO2C6H4C(dO)SC6H4Z.11e Sen-
atore et al.2b,17 obtained a biphasic rate dependence on
the basicity of nucleophiles in the reaction of alkoxides
and phenoxides with aryl methanesulfinates (CH3S(dO)-
OAr), which indicated a mechanistic changeover from a
breakdown (âX ) 0.79) to formation (âX = 0.0) of TBP
intermediate as the basicity of the nucleophile is in-
creased. The âX of near zero is even smaller than the
values obtained (âX ) 0.09-0.14) with the strongly basic
nucleophiles in Table 2. Since in the rate-limiting forma-
tion of T( the central atom (P)-leaving group (OAr) bond
should not break but relax somewhat, the âZ values
should be much smaller than those in the concerted
processes where the leaving group bond is mostly broken
and the nucleophile-phosphorus bond is mostly formed
in the TS. (iv) The unusually large negative FXZ value
for the weakly basic nucleophiles (FXZ ) -1.98) is indica-
tive of a TS formed by the concerted front-side nucleo-
philic attack,18 II, since the nucleophile and leaving group
can be in close proximity in such TS. Although we have
no conclusive proof for this proposal, there are evidence18b
in support of such structural dependence of the large
negative FXZ as we have advanced in connection with the
frontside attack SN2 TS structures for the anilinolysis of
1- (FXZ ) -0.56)19a and 2-phenylethyl (FXZ ) -0.45)19b and
pKa(H2O) ) (-2.11 ( 0.08)σ - + 9.89 ( 0.05;
r ) 0.960, N ) 8 (5a)
pKa(MeCN) ) (-4.35 ( 0.09)σ - + 26.58 ( 0.09;
r ) 0.998, N ) 12 (5b)
Regarding the biphasic linear free energy relationships
with nucleophile variation, FX (Fnuc) and âX (ânuc), we
propose mechanistic change from a concerted (with less
basic pyridines) to a stepwise mechanism with rate-
limiting formation of a TBP-5C intermediate, T( (with
more basic pyridines), based on the following grounds:
(i) It is generally known that the concerted path is
favored by weakly basic nucleophiles while the stepwise
path is favored by strongly basic nucleophiles.4,9 (ii) The
sign of FXZ is negative (FXZ ) -1.98 for weakly basic
nucleophiles) for the concerted paths but is positive for
the stepwise processes (FXZ ) 0.97 for strongly basic
nucleophiles) in accordance with the mechanistic criteria
established based on theoretical10 as well as experimental
results.5,11 (iii) The magnitudes of âX and âZ values are
relatively large for weakly basic nucleophiles (âX ) 0.22-
0.39, âZ ) -0.42 to -0.56). Although the âX values in the
range 0.4-0.7 are obtained for most of the concerted
displacement reactions at the carbonyl carbon12 and
phosphoryl P centers,13 some âX values smaller than 0.4
(7) Albert, A.; Serjeant, E. P. The Determination of Ionization
Constants, 3rd ed.; Chapman and Hall: New York, 1984.
(8) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic
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1990, 112, 8115. (b) Williams, A. Acc Chem. Res. 1989, 22, 387.
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133.
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2 1993, 2411. (b) Koh, H. J .; Kim, S. I.; Lee, B. C.; Lee, I. J . Chem.
Soc., Perkin Trans. 2 1996, 1353. (c) Koh, H. J .; Lee, J . W.; Lee, H.
W.; Lee, I. New J . Chem. 1997, 21, 447. (d) Koh, H. J .; Lee, J . W.; Lee,
H. W.; Lee, I. Can. J . Chem. 1998, 76, 710. (e) Koh, H. J .; Han, K. L.;
Lee, I. J . Org. Chem. 1999, 64, 4783. (f) Lee, I.; Koh, H. J . New J .
Chem. 1996, 20, 131. (g) Koh, H. J .; Kim, T. H.; Lee, B.-S.; Lee, I. J .
Chem. Res., Synop. 1996, 482; J . Chem. Res., Miniprint 2741. (h) Oh,
H. K.; Yu, J . H.; Cho, I. H.; Lee, I. Bull. Korean Chem. Soc. 1997, 18,
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(14) (a) Skoog, M. T.; J encks, W. P. J . Am. Chem. Soc. 1984, 106,
7697. (b) Bourne, N.; Williams, A. J . Am. Chem. Soc. 1984, 106, 7591.
(c) Williams, A. J . Am. Chem. Soc. 1985, 107, 6335. (d) Ba-Saif, S. A.;
Waring, M. A.; Williams, A. J . Chem. Soc., Perkin Trans. 2 1991, 1653.
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1989, 111, 2647.
(16) (a) Gresser, M. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99,
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M. G.; Salinas, S.; Santos, J . G. J . Org. Chem. 1999, 64, 4817. (e)
Butler, A. R.; Robertson, I. H.; Bacaloglu, R. J . Chem. Soc., Perkin
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63, 6820. (b) Castro, E. A.; Leandro, L.; Millan, P.; Santos, J . G. J .
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(13) (a) Bourne, N.; Chrystiuk, E.; Davis, A. M.; Williams, A. J . Am.
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2218 J . Org. Chem., Vol. 67, No. 7, 2002
Lee et al.
Ta ble 3. Com p a r ison of Bon d Len gth (Å) in P h osp h a tes
w ith Y (F or Cl) a n d R (CH3 or OCH3) a s Liga n d s
Ca lcu la ted a t th e B3LYP /6-311+G** Level
Ta ble 4. MO Levels (E in Au ) for 3 a n d 4 Ca lcu la ted a t
th e NBO-HF /6-311+G**//B3LYP /6-311+G** Level
Y
MO
ꢀ (au)
F
nO(sp2)
nO(p)
σ(P-F)
σ*(P-F)
nO(sp2)
nO(p)
-0.8215
-0.5446
-1.1712
+0.6589
-0.8188
-0.5471
-0.8056
+0.2268
Cl
TBP-5C adduct
σ(P-Cl)
σ*(P-Cl)
reactant
(2A) [F2PO(CH3)2]-
(2) FPO(CH3)2
ax
eq
∆d(ax - eq)
a
dP-C
dP-F
1.811
1.619
1.895
1.828
1.845
1.752
+0.050
+0.076
(3A) [F2PO(OCH3)2]-
(3) FPO(OCH3)2
ax
eq
∆d(ax - eq)
a
dP-O
dP-F
1.585
1.599
1.695
1.736
1.663
1.714
+0.032
+0.022
(4A) [Cl2PO(OCH3)2]-
(4) ClPO(OCH3)2
ax
eq
∆d(ax - eq)
a
dP-O
dP-Cl
1.590
2.074
1.660
2.487
1.599
2.398
+0.061
+0.089
a
Distance between P and R.
cumyl arenesulfonates (FXZ ) -0.75).19c Frontside attack
by the nucleophile should lead to retention of configura-
tion as shown in II. (v) The relatively smaller magnitudes
of FZ and FXZ values for the stronger nucleofuge (or less
basic phenoxides) with Z ) 3-CN, 4-NO2 suggest backside
nucleophilic attack since the more apicophilic (or the
stronger) the leaving group, the lower the energy is for
the concerted path, i.e., the more likely is the backside
attack with an ap(NX)sap(LZ) TS structure, I, ac-
companied by an inversion of configuration.4 In this
structure, both the nucleophile and leaving group are on
the apical sites so that both the XN-P and P-LZ bonds
are longer than those for an equatorial site. The longer
distance involved in the TS leads to smaller magnitudes
F igu r e 1. Plot of log k2 vs pKa (X-pyridines, in H2O) for the
reactions of Z-aryl bis(4-methoxyphenyl) phosphates with
X-pyridines in acetonitrile at 55.0 °C.
of both FZ, âZ and FXZ.
5,20 It is known that pentacoordinate
species have apical bonds that are significantly longer
than regular single bonds.4,13a,21
In Table 3, bond length changes are compared for three
selected model systems, FPO(CH3)2, (2), FPO(OCH3)2, (3),
and ClPO(OCH3)2, (4), calculated at the B3LYP/6-
311+G** level.22 These three systems represent those
without lone pair for n - σ* charge transfer change (2),
and with lone pair (n) for the n-σ* charge-transfer
interaction between n and σ* orbitals of axial bonds (3
and 4); in the former (3), the σ/P-F level is very much
F igu r e 2. Plot of log k2 vs pKa (Z-phenols, in H2O) for the
reactions of Z-aryl bis(4-methoxyphenyl) phosphates with
X-pyridines in acetonitrile at 55.0 °C.
higher (σ/P-F ) 0.6589 au) than the σ/P-Cl level (σ/P-Cl
)
0.2268 au) in the latter (4), Table 4. In all cases, all of
the bonds including the PdO bond stretch in the TBP-
5C adduct where PdO becomes a single bond (P-O-).
For example, the P-F bond in (2) extends by more than
0.2 Å on going from the reactant to the axial P-F bond
in the TBP-5C adduct. Again we note that the axial bonds
are longer than the corresponding equatorial bonds,
especially for dP-Cl the difference is the greatest with
∆d(ax-eq) ) 0.089 Å. In other words, the lower the σ*
level, the longer is the axial than the equatorial bond
(vide infra).
(20) Lee, I. J . Phys. Org. Chem. 1992, 5, 736.
Our density functional theory (DFT) results at the
B3LYP/6-311+G** level22 show that the TBP-5C adducts,
(2A) ∼ (4A) in Figure 3, have axial P-Y (where Y ) F or
Cl) conformations. Interestingly, however, the p lone pair
(nO(p)) on the equatorial methoxy oxygen atom in (3A)
adopts a preferred orientation that is perpendicular ( )
to the apical axis, III, in contrast that in (4A) adopts the
(21) (a) Perozzi, E. F.; Martin, J . C.; Paul, I. C. J . Am. Chem. Soc.
1975, 96, 6735. (b) Ramirez, F. Acc. Chem. Res. 1968, 1, 168. (c)
McDowell, R. S.; Streitwieser, A. J . Am. Chem. Soc. 1985, 107, 5849.
(d) Lee, I.; Kim, C. K.; Lee, B.-S.; Ha, T.-K. THEOCHEM 1993, 279,
191.
(22) (a) Becke, A. D. J . Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (c) Foresman, J . B.; Frisch,
Æ. Exploring Chemistry with Electronic Structure Methods, 2nd ed.;
Gaussian Inc.: Pittsburgh, 1993.

Substitution Reactions of Aryl Bis(4-methoxyphenyl) Phosphates
J . Org. Chem., Vol. 67, No. 7, 2002 2219
F igu r e 3. Structures of reactants 2-4 and TBP-5C adducts (2A-4A calculated at the B3LYP/6-311+G** level (bond lengths in
Å, and angles in deg).
2
conformation in which the p orbital axis is parallel (|)
with the apical axis, IV.
2 < σ|F|σ* >
(2)
σ-σ*
∆E
) -
(6)
ꢀσ/ - ꢀσ
This was ascribed to the following: (i) larger n( ) -
σ/P-He charge-transfer energies (eq 6) than the n(|) -
σ/P-Ha interaction energies23c due to the higher σ/P-Ha
(wider ∆ꢀ ) ꢀσ/ - ꢀn in eq 6) rather than σ/P-He level even
though overlap between n(|) and P-Ha is known to be
greater (and hence the matrix element, Fnσ*, is greater
in eq 6) than that between n( ) and P-He; and (ii) larger
exclusion repulsion energies21a,23b,25 included in the no-
The bonding in PH4NH2, where N has a p lone pair
nN(p), has been discussed by many authors.21,23 In this
charge-transfer term (∆ENCT) in eq 724b where ∆ECT is
compound, V, the second-order perturbation energy,
(2)
σ-σ*
(2)
σ-σ*
the total σ-σ* charge-transfer energies ∆E
in eq 6.
∆E
in eq 624 where σ and σ* represent bonding (n, π,
The ∆ENCT term includes also energies due to electrostatic
(induction and polarization) interactions24b and is the
Lewis energy, ELew, associated with the localized Har-
tree-Fock wave function26 (corresponding essentially to
or σ) and antibonding (π* or σ*) orbitals, respectively, in
general and F is the Fock operator, has been shown to
be much more stabilizing in perpendicular ( ) nN(p) form
rather than in parallel (|) nN(p) form.21a,23,25
a Lewis structure). The σAB-σ/ NBO delocalization
CD
leads to a decrease in A-B and C-D bond orders (and
hence weaken A-B and C-D bonds) and a simultaneous
increase in B-C bond order24 (strengthen B-C bond).
(23) (a) Strich, A.; Veillard, A. J . Am. Chem. Soc. 1973, 95, 5574.
(b) Wang, P.; Zhang, Y.; Glaser, R.; Reed, A. E.; Schleyer, P. v. R.;
Streitwieser, A. J . Am. Chem. Soc. 1991, 113, 55.
(24) (a) Epiotis, N. D.; Cherry, W. R.; Shaik, S.; Yates, R. L.;
Bernardi, F. Structural Theory of Organic Chemistry; Springer-
Verlag: Berlin, 1977; Part I. (b) Reed, R. E.; Curtiss, L. A.; Weinhold,
F. Chem. Rev. 1988, 88, 899. (c) Gledening, E. D.; Weinhold, F. J .
Comput. Chem. 1988, 19, 610. (d) Glendening, E. D.; Badenhoop, J .
K.; Weinhold, F. J . Comput. Chem. 1998, 19, 628.
(25) Hoffmann, R.; Howell, J . M.; Muetterties, E. L. J . Am. Chem.
Soc. 1972, 94, 3057.
(26) Carballeira, L.; Perez-J uste, I. J . Phys. Chem. A 2000, 104,
9362.

2220 J . Org. Chem., Vol. 67, No. 7, 2002
Lee et al.
leading to relatively strong nO-σ/ proximate charge-
∆E ) E(TBP-5C) - E(separated reactants) )
ap
transfer interactions.24 As a result of this charge transfer
∆ECT + ∆ENCT (7)
The bond-bond (σ-σ) four-electron (exclusion) repul-
sion interaction is greater in the n(|)-σP-Ha interaction
since overlap between the two interacting orbitals is
greater than that in the n( )-σP-He interaction. The σ-σ
four-electron repulsion is strongly dependent on the
overlap between the two interacting orbitals.24a These two
factors are also responsible for the preferred n( ) and n(|)
conformations of the oxygen p lone pair in 3A and 4A,
respectively (Figure 3). For 3A, the level gap is ∆ꢀ )
ꢀσ/ - ꢀn ) 1.2035 au, whereas that for 4A is ∆ꢀ ) 0.7739,
and hence, the n-σ* charge-transfer stabilization ener-
gies in 3A should be much smaller than that in 4A due
to the much larger energy gap in eq 6. On the other hand,
however, since the overlap in the n(|) forms is much
greater and hence the exclusion repulsion destabiliza-
tion should be much greater in this form. Due to the
large destabilization of n(|)-σP-F and smaller n(|)-σP/ -F
charge-transfer stabilization, that are expected from the
results on PH4NH2 (vide supra), 3 will be forced to take
n( ) structure of 3A. In contrast for 4, the n(|)-σP/ -Cl
interaction energies are so strong that they can over-
whelm the n(|)-σP-Cl repulsive destabilization.
from the lone pairs to the apical σ* bonds, the two apical
bonds are stretched and weakened so that apical bond
cleavage is facilitated. The apical bond to the attacking
nucleophile should also be weaker than the corresponding
equatorial bond. Thus, in the ap(NX)sap(LZ) direct
attack processes, the magnitudes of FZ (and âZ) and FXZ
values will be small due to long distances involved in the
apical bonds of the TBP-5C type TS. This is quite similar
to the enhanced leaving group expulsion from of the
tetrahedral intermediate (VII) by an alkoxy (OR) or
phenoxy (OPh) ligand, as time and again pointed out by
Castro et al.16b,27 qualitatively, in the nucleophilic car-
bonyl substitution reactions. For example, Castro et al.12e
observed that the aminolysis of S-(2,4-dinitrophenyl)
O-ethyl carbonate (5) with secondary alicyclic amines
proceeds by a concerted mechanism with âX ) 0.56,
whereas S-(2,4-dinitrophenyl) acetate (6) reacts by a
stepwise mechanism with âX ) 0.8-1.0.
These expectation are borne out in the results of our
NBO analysis24b-d at the HF/6-311+G** level with the
B3LYP/6-311+G** geometries. For 3A, the adduct with
n( ) form is more stable than the n(|) form by δ∆E )
∆E( ) - ∆E(|) ) -7.3 kcal mol-1. Decomposition of this
energy difference into δ∆ECT and δ∆ENCT in eq 7 leads
to
a
greater exclusion repulsion destabilization
(∆ENCT > 0) of δ∆ENCT () -15.5 kcal mol-1) for the n(|)
form. The charge-transfer stabilization (∆ECT < 0) is also
larger (δ∆ECT ) +8.2 kcal mol-1) for the n(|) than the
n( ), but the difference is smaller than that for δ∆ENCT
.
Thus, despite the larger charge-transfer stabilization
with n(|) than n( ) form, even greater destabilization
caused mainly by the exclusion repulsion with n(|) form
results in the overall stability of the n( ) relative to n(|)
adduct for 3A. In contrast, the overall stability of the n(|)
relative to the adduct for 4A (by 5.2 kcal mol-1) is due
to the greater charge-transfer stabilization (δ∆ECT
)
25.9 kcal mol-1) with smaller repulsive interaction
(δ∆ENCT ) -20.7 kcal mol-1) for the n(|) than n( ) form.
Both the nO-σ/
(-36.2 kcal mol-1) and the overall
P-Cl
charge transfer (-106.7 kcal mol-1) stabilization for Y )
Cl are greater than nO-σ/ (-28.5 kcal mol-1) and the
P-F
overall delocalization (-87.8 kcal mol-1) stabilization for
Y ) F in the n(|) forms. This is, of course, the narrower
energy gap ∆ꢀ ) ꢀσ/ - ꢀn with Y ) Cl than with Y ) F in
eq 6, which in turn is primarily due to the lower σP/ -Cl
than σ/P-F level (Table 4).
In the putative tetrahedral intermediate (T() that may
be formed in the aminolysis reactions of 5, the leaving
ability becomes so strong due to weakening of the CsLZ
bond that the intermediate cannot exist and the reaction
proceeds by a concerted path. This enhanced leaving
ability in the tetrahedral adduct with 5 compared to that
These model system analyses show that: (i) The p lone
pair on the atom directly attached to the central P atom
has important stabilizing effect on the TBP-5C structure
with P-Y apical bond where Y is an electronegative
atom. (ii) The lower the σ/P-Y orbital, the greater is the
stability of n(|) form, and hence the greater is the
apicophilicity of Y.
with 6 should result from nO-σ/
type proximate
C-S
charge-transfer interaction (VII).24 The nO-σ/
type
C-LZ
interaction should be the greater, the higher the lone pair
n (or bonding orbital, σ) energy level and the lower the
In the TBP-5C adduct of aryl bis(4-methoxyphenyl)
phosphates, 1, the orientations of lone pairs (p-type) on
the two directly bonded oxygens are most probably
parallel with the two apical bonds of phosphorus (VI)
(27) (a) Castro, E. A.; Pavez, P.; Santos, J . G. J . Org. Chem. 2001,
66, 3129. (b) Castro, E. A.; Ibanez, F.; Santos, J . G. J . Org. Chem. 1992,
57, 7024.

Substitution Reactions of Aryl Bis(4-methoxyphenyl) Phosphates
J . Org. Chem., Vol. 67, No. 7, 2002 2221
Ta ble 5. Activa tion P a r a m eter sa for th e Rea ction s of
Z-Ar yl Bis(4-m eth oxyp h en yl) P h osp h a tes w ith
X-P yr id in es in Aceton itr ile
from a concerted (FXZ ) -1.98) to a stepwise (FXZ ) +0.97)
process as the basicity of nucleophile becomes stronger.
The magnitude of Bro¨nsted âZ (âlg) values for the better
leaving groups (Z)3-CN, 4-NO2) are unusually low
(âZ ) -0.01 to -0.03) suggesting involvement of a direct
backside attack TS in which both the nucleophile (NX)
and leaving group (LZ) occupy apical sites (ap(NX)sap-
(LZ)). The putative TBP-5C adduct leads to a concerted
mechanism due to strong proximate charge transfer
interactions between the p-lone pair on the equatorially
bonded oxygen and the apical bond, (nO(eq)sσ*(ap)).
These are supported by the results of NBO analyses at
the NBO-HF/6-311+G**//B3LYP/6-311+G** level of
theory.
k2 × 104
∆Hq
-∆Sq
Z
X
T/°C (M-1 s-1
)
(kcal mol-1
)
(cal mol-1 K-1
)
4-Cl
4-CH3
45
55
65
45
55
65
45
55
65
14.4
24.0
37.0
5.90
8.55
11.7
2.67
2.88
3.20
9.4 ( 0.2b
6.6 ( 0.2
1.3 ( 0.2
33 ( 1c
43 ( 1
62 ( 1
4-CN
H
4-NO2 3-Cl
a
Calculated by Eyring equation. b,c Standard deviation.
CsLZ σ* level because the denominator in eq 6, ∆ꢀ )
ꢀσ/ - ꢀσ, decreases. Since the ethyl group in 5 is an
electron donor, the lone pair level on ethoxy oxygen is
raised,2b,28 and electron-withdrawing groups on the leav-
Exp er im en ta l Section
Ma ter ia ls. Aryl bis(4-methoxyphenyl)phosphates were pre-
pared by following two steps. In step 1, bis(4-methoxyphenyl)-
chlorophosphate was prepared by reacting phosphorus oxy-
chloride with 4-methoxyphenol for 2 h in the presence of
triethylamine in methylene chloride on an ice bath.29a Tri-
ethylamine hydrochloride salt was separated by filtration. The
remaining product was treated with ether and water for
workup. After workup, anhydrous MgSO4 was added, and the
solvent was evaporated under reduced pressure after filtration.
Bis(4-methoxyphenyl) chlorophosphate was identified by TLC,
ing group (2,4-(NO2)2) lower the σ/ level2b,28 leading to
C-S
a stronger charge transfer by a reduced ∆ꢀ in eq 6. In
the tetrahedral structure VII, however, the overlap
between the p-type lone pair on the oxygen and the bond
to the leaving group (CsLZ) can be only partial due to
the relative orientations within the tetrahedral structure,
i.e., Fσσ/ ( Sσσ/) may be small in eq 6. The corresponding
overlap in the TBP-5C adduct VI is much more efficient
with nearly parallel orientations of the p-lone pair to the
apical bonds. Therefore a good leaving group, which has
low σ/P-LZ level due to stronger electron-acceptor sub-
stituent Z, will have a strong apicophilicity, and is likely
to react by a backside attack concerted mechanism with
inversion of configuration. In contrast a poorer leaving
group with a weaker apicophilicity is more likely to
occupy an equatorial site in the TBP-5C adduct and
pseudorotates to an apical site before bond cleavage.
Activation parameters, ∆Hq and ∆Sq, were determined
for 3 cases as shown in Table 5. The activation enthalpies
are relatively low, especially for the reactions with good
leaving groups, Z ) 4-CN and 4-NO2. For these two
reactions, the leaving group departure seems very facile
and has progressed to a large extent in the TS as low
enthalpies of activation and large negative entropies of
activation suggest.
IR, H NMR, 13C NMR, and GC-MS analysis. In step 2, aryl
1
bis(4-methoxyphenyl) phosphates were synthesized by reacting
bis(4-methoxyphenyl)chlorophosphate with phenols in the
presence of triethylamine in methylene chloride. The sub-
strates were isolated in the similar way described in step 1
and were identified by TLC, IR, 1H NMR, 13C NMR, and GC-
MS analysis. All other materials were as described previ-
ously.29 Aldrich GR Grade pyridines were used without
purification. The physical constants after column chromatog-
raphy (silica gel/ethyl acetate + n-hexane) were as follows:
Bis(4-m eth oxyp h en yl) ch lor o p h osp h a te: liquid; δH
(CDCl3) 6.8-7.3 (bis-4-CH3OC12H8-H, 8H, m), 3.8 (OCH3-H,
3H, s); δC (CDCl3) 113-120 (CdC, aromatic), 54 (OCH3); νmax
(neat) 3073 (C-H, aromatic), 2838 (C-H, aliphatic), 1598,
1506, 1465, 1255 (POC6H4), 1312 (PdO); m/z 329 (M+).
4-Ch lor op h en yl bis(4-m eth oxyp h en yl) p h osp h a te: liq-
uid; δH (CDCl3) 7.0-7.1 (bis-4-OCH3C12H8-H + 4-ClC6H4-H,
12H, m), 3.8 (OCH3, 3H, s); δC (CDCl3) 113-128 (CdC,
aromatic), 54 (OCH3); νmax (neat), 3070 (CH, aromatic), 2839
(CH, aliphatic), 1598, 1501, 1254, 1184 (POC6H4), 1308 (Pd
O); m/z 421 (M+)
3-Ch lor op h en yl bis(4-m eth oxyp h en yl) p h osp h a te: liq-
uid; δH (CDCl3) 6.7-7.3 (bis-4-CH3OC12H8-H + 3-ClC6H4-H,
12H, m), 3.7 (OCH3, 3H, s); δC (CDCl3) 113-127 (CdC,
aromatic), 54 (OCH3); νmax (neat) 3072 (CH, aromatic), 2840
(C-H, aliphatic), 1595, 1501, 1254, 1183 (P-O-C6H4), 1307
(PdO); m/z 421 (M+).
3-Cya n op h en yl bis(4-m eth oxyp h en yl) p h osp h a te: Liq-
uid δH (CDCl3) 6.7-7.5 (bis-4-CH3OC12H8-H + 3-CNC6H4-H,
12H, m), 3.8 (OCH3, 3H, s); δC (CDCl3) 113-119 (CdC,
aromatic), 54 (OCH3); νmax (neat) 3001 (CH, aromatic), 2839
(CH, aliphatic), 2225 (CtN), 1614, 1503, 1448, 1180 (POC6H4),
1300 (PdO); m/z 412 (M+).
4-Cya n op h en yl bis(4-m eth oxyp h en yl) p h osp h a te: liq-
uid; δH (CDCl3) 6.7-8.0 (bis-4-CH3OC12H8-H + 4-CNC6H4-H,
12H, m), 3.8 (OCH3, 3H, s); δC (CDCl3) 113-132 (CdC,
aromatic), 54 (OCH3); νmax (neat), 3000 (C-H, aromatic), 2838
(C-H, alipahtic), 2227 (CtN), 1614, 1502, 1447, 1180 (POC6H4),
1297 (PdO); m/z 412 (M+).
4-Nitr op h en yl bis(4-m eth oxyp h en yl) p h osp h a te: liquid;
δH (CDCl3) 6.8-8.2 (bis-4-CH3OC12H8-H + 4-NO2C6H4-H, 12H,
m), 3.8 (OCH3, 3H, s); δC (CDCl3) 113-124 (CdC, aromatic),
Con clu sion
The rates of the pyridinolysis of Z-aryl bis(4-meth-
oxyphenyl) phosphates in acetonitrile show a biphasic
dependence on the basicity of the pyridine nucleophiles.
The Bro¨nsted coefficiant âX (ânuc) changes from a large
(âX ) 0.22-0.39) to a small (âX ) 0.09-0.14) value as
the basicity of pyridine increases. The magnitude of the
Bro¨nsted coefficient âZ (âlg) shows also similar changes
from a large (âZ ) -0.42 to -0.56) to a small value (âZ )
-0.08 to -0.28) for the poorer leaving groups (Z ) 4-Cl,
3-CN). These results are interpreted to indicate a mecha-
nistic change from a concerted for the weakly basic
nucleophiles (X ) 3-Cl, 4-CN) to a stepwise mechanism
with rate-limiting formation of the TBP-5C transition
state for the strongly basic nucleophiles (X ) 4-NH2,
3-CH3). The sign change of the cross-interaction con-
stants, FXZ, supports the proposed mechanistic change
(29) (a) Guha, A. K.; Lee, H. W.; Lee, I. J . Chem. Soc., Perkin Trans.
2 1999, 765. (b) Guha, A. K.; Lee, H. W.; Lee, I. J . Org. Chem. 2000,
65, 12.
(28) Fleming, I. Frontier Orbitals and Organic Chemical Reactions;
Wiley: London, 1976; Chapter 4.

2222 J . Org. Chem., Vol. 67, No. 7, 2002
Lee et al.
54 (OCH3); νmax 3114 (CH, aromatic), 2838 (CH, aliphatic),
1588 and 1347 (NO2), 1527, 1506, 1347, 1194 (POC6H4), 1296
(PdO); m/z 430 (M+).
Kin etic P r oced u r e. Rates were measured as described
previously6g,29 with a large excess of pyridines: [substrate] )
1 × 10-3 M and [pyridines] ) 0.04-0.16 M.
were confirmed by calculations of vibrational frequencies. The
natural bond orbital (NBO) analyses24b-d were carried out to
obtain proximate bond-antibond (σ-σ*) orbital interaction
energies at the HF/6-311+G** level.
Ack n ow led gm en t. This work was supported by
Grant No. R01-1999-00047 from the Basic Research
Programs of the Korea Science and Engineering Foun-
dation.
P r od u ct An a lysis. 4-Chlorophenyl bis(4-methoxyphenyl)
phosphate was refluxed with excess 4-acetylpyridine for more
than 15 half-lives at 55.0 °C in acetonitrile. Acetonitrile was
evaporated under reduced pressure, and the product mixture
was treated with ether and water for workup; during workup,
dilute hydrochloric acid was treated to remove excess 4-acetyl-
pyridine and after workup dried over anhydrous MgSO4. The
product was isolated by evaporating the solvent under reduced
pressure after filtration. The physical constants after column
chromatography (silica gel/ethyl acetate + n-hexane) were as
follows. (4-CH3OC6H4O)2P(dO)N+C5H4-4-COCH3: liquid; δH
(CDCl3) 6.6-7.3 (bis-4-methoxy-C12H8-H + 4-COCH3C5H4-H,
12H, s), 3.8 (OCH3, 3H, s), 1.7 (COCH3, 3H, s); νmax (neat) 3076
(CH, aromatic), 2839 (CH, aliphatic), 1872 (CH3CO), 1727
(CO), 1588, 1502, 1029 (POC6H4), 1303 (PdO).
Su p p or tin g In for m a tion Ava ila ble: The calculated elec-
tronic energies and Z-matrixes of 2, 2A-axial, 2A-equatorial,
3, 3A-axial, 3A-equatorial, 4, 4A-axial, and 4A-equatorial. This
material is available free of charge via the Internet at
http://pubs.acs.org.
J O0162742
(30) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen, W.;
Wong, M. W.; Andres, J . L.; Gonzalez, C.; Head Gordon, M.; Replogle,
E. S.; Pople, J . A. Gaussian 98, Revision A.6. Gaussian, Inc.: Pitts-
burgh, PA, 1998.
Ca lcu la tion s. Density functional theory (DFT) calculations
were performed with Gaussian 98 system of programs.30 The
DFT method is particularly useful for calculations involving
large number of nonhydrogen atoms at the correlated level.
The B3LYP hybrid exchange-correlation functionals are known
to give good equilibrium geometries, vibrational frequencies
and accurate molecular atomization energies.31 Geometries
were optimized at the B3LYP/6-311+G** level of theory. To
represent lone pair orbitals and second row elements (P, Cl)
(also H) properly a diffuse function (+) and polarization
functions (**) were used, respectively. Equilibrium structures
(31) Levine, I. N. Quantum Chemistry, 5th ed.; Prentice Hall: Upper
Saddle River, NJ , 2000; p 573.