ARTICLE pubs.acs.org/JPCA
Structures, Energies, and Spin Spin Coupling Constants of Methyl-Substituted 1,3-Diborata-2,4-diphosphoniocyclobutanes: Four-member B P B P Rings B2P2(CH3)nH8 n, with n = 0, 1, 2, 4 Janet E. Del Bene,*,† Ibon Alkorta,*,‡ and Jose Elguero‡ † ‡
Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555, United States Instituto de Química Medica (CSIC), Juan de la Cierva, 3, 28006-Madrid, Spain
bS Supporting Information ABSTRACT: An ab initio study has been carried out to determine the structures, relative stabilities, and spin spin coupling constants of a set of 17 methylsubstituted 1,3-diborata-2,4-diphosphoniocyclobutanes B2P2(CH3)nH8 n, for n = 0, 1, 2, 4, with four-member B P B P rings. The B P B P rings are puckered in a butterfly conformation, in agreement with experimental data for related molecules. Isomers with the CH3 group bonded to P are more stable than those with CH3 bonded to B. If there is only one methyl group or if two methyl groups are bonded to two different P or B atoms, isomers with equatorial bonds are more stable than those with axial bonds. However, when two methyl groups are present, the gem isomers are the most stable for molecules B2P2(CH3)2H6 with P C and B C bonds, respectively. Transition structures present barriers to the interconversion of two equilibrium structures or to the interchange of axial and equatorial positions in the same isomer. These barriers are very low for the isomer with two methyl groups bonded to B in axial positions for the isomer with four axial bonds and for the isomer with geminal B C bonds at both B atoms. Coupling constants 1J(B P), 1J(P C), 1 J(B C), 2J(P P), and 3J(P C) are capable of providing structural information. They are sensitive to the number of methyl groups present and can discriminate between axial, equatorial, and geminal bonds, although not all do this to the same extent. The one-bond coupling constants 1J(B P), 1J(P C), and 1J(B C) are similar in equilibrium and transition structures, but 3J(P C) and 2J(P P) are not. These coupling constants and those of the corresponding fluoro-derivatives of the 1,3-diborata-2,4-diphosphoniocyclobutanes demonstrate the great sensitivity of phosphorus coupling to structural and electronic effects.
’ INTRODUCTION Computational quantum and experimental chemistry appear to be approaching asymptotically, never to completely coincide. As the capability of carrying out accurate calculations on larger molecules increases, the complexity of problems being investigated experimentally also increases. As a result, it is often necessary for theory to examine simplified models of experimentally interesting systems. Recently, we employed this approach to examine molecules A D, illustrated in Scheme 1, which were synthesized and characterized by Bertrand in his continuing investigations of molecules with B P B P rings.1 To do this, we removed the large substituents (iso-propyl, tert-butyl, adamantly, etc.), which are essential for stabilizing the B P B P rings (called the corset effect2) and replaced them with fluorine atoms. This allowed us to investigate fluorine-substituent effects on the structures, relative stabilities, and spin spin coupling constants of four-member B P B P rings in molecules B2P2FnH8 n, for n = 0, 1, 2, and 4.3 Several of these molecules are illustrated in Scheme 2. As an extension of that study, we have replaced the fluorine atoms with methyl groups in molecules B2P2(CH3)nH8 n, for n = 0, 1, 2, and 4. Given the very different electronic effects of F r 2011 American Chemical Society
and CH3 as substituents, methyl substitution will have very different effects on the properties of these four-member B P B P rings. Moreover, replacing the bulky organic substituents in molecules A D by methyl groups provides a better model for the molecules illustrated in Scheme 1. In this paper we report the structures, binding energies, and spin spin coupling constants for these methyl derivatives, and compare these properties with those of the corresponding fluoro-substituted derivatives reported in ref 3.
’ METHODS The structures of the methyl-substituted B P B P rings were optimized at second-order Møller Plesset perturbation theory (MP2)4 7 with the 6-31+G(d,p)8 11 basis set. Frequency calculations were carried out at this level to confirm that the optimized structures correspond to either equilibrium (no imaginary frequencies) or transition (1 imaginary frequency) structures. These structures were subsequently reoptimized at Received: July 16, 2011 Revised: August 23, 2011 Published: September 07, 2011 10502
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Scheme 1. Four 1,3-Diborata-2,4-diphosphoniocyclobutanes Described in Ref 1d
Scheme 2. Selected Fluoro-Substituted 1,3-Diborata-2,4-diphosphoniocyclobutanes from Ref 3
MP2 with the larger aug-cc-pVTZ basis set,12,13 and the MP2/ aug-cc-pVTZ structures were used for the coupling constant calculations. The optimization and frequency calculations were carried out using Gaussian 03.14 The molecules included in this study which have one or two methyl groups have been restricted to those with Cs or higher symmetry, because coupling constant calculations with the method specified below are not feasible for methyl-substituted B P B P rings with only C1 symmetry. For molecules with four methyl groups, C2v or higher symmetry has been required for the same reason. As a result, except for molecules 15 and 17, which have one methyl group bonded to each B and P in equatorial (Mealleq) or axial (Meallax) positions, the remaining molecules have methyl groups bonded to either P (14, PMegem, PMegem) or B (16 BMegem,BMegem), but not to both in the same molecule. Spin spin coupling constants were evaluated using the equation-of-motion coupled cluster singles and doubles method (EOM-CCSD) in the CI (configuration interaction)-like approximation,15,16 with all electrons correlated. The Ahlrichs qzp basis set was placed on 13C and the qz2p basis on 31P.17 Because an Ahlrichs qzp basis is not available for B, a corresponding basis set which had been constructed and used previously for studies of B H, B N, and B Li coupling, was placed on 11B.18 The Dunning cc-pVDZ basis set was placed on 1H, and no coupling constants involving H are reported. In the Ramsey approximation, the total coupling constant (J) is expressed as a sum of four terms: the paramagnetic spin orbit (PSO), diamagnetic spin orbit (DSO), Fermi contact (FC), and spin-dipole (SD).19 The EOM-CCSD calculations were carried out with ACES II20 on the IBM cluster 1350 (Glenn) at the Ohio Supercomputer Center.
’ RESULTS AND DISCUSSION The molecules investigated in this study include the parent molecule B2P2H8, four isomers B2P2(CH3)1H7, eight isomers B2P2(CH3)2H6, and four isomers B2P2(CH3)4H4. These are illustrated in Figure S1 of the Supporting Information. Total energies of equilibrium and transition structures are given in Table S1 of the Supporting Information, and geometries are reported in Table S2. The numbering of P and B atoms is
illustrated in Figure 1. All of these molecules have either Cs or C2v symmetry with puckered rings in a butterfly conformation and axial and equatorial B C and P C bonds. The molecules are identified by number and are distinguished by the number and positions of the methyl groups (designated Me), as indicated in Table 1. If there is only one methyl group bonded to a P atom or two methyl groups bonded to the same P, that atom is designated P1, as illustrated for molecule 3 in Figure 1; if there is only one methyl group bonded to a B atom or two methyl groups bonded to the same B, it is B2. Whenever necessary, molecules are named so as to avoid ambiguity, as for example, molecule 8 in Figure 1 is P1Meax,P3Meeq to indicate an axial P1-CH3 bond and an equatorial P3-CH3 bond. Structures. In our previous paper3 we noted several difficulties in attempting to compare computed equilibrium structures to experimental ground-state structures. Among these are the different substituents, methyl groups in the computed structures and the large bulky organic groups in the experimental ones; the different phases, gas-phase for the computed structures and crystal structures for the experimental ones; and the neglect of zero-point vibrational motion for the computed structures. However, all of the computed structures have puckered rings, in agreement with the experimental structures. The computed B P distances range from 1.96 to 2.02 Å, consistent with the experimental range of 1.97 to 2.01 Å. Experimental B B distances range from 2.80 to 2.84 Å, while the computed values give a wider range from 2.79 to 2.92 Å. The experimental P P distances range from 2.79 to 2.84 Å, while the computed range is about 0.1 Å shorter, from 2.65 to 2.73 Å. Because the ring inversion barriers can be relatively low in some molecules, zeropoint motion tends to produce rings that approach a more nearly planar structure, and this motion would increase the upper values of both the B B and the P P distances. Energies. Table 1 reports the energies of molecules 1 17 relative to the lowest-energy isomer with a given number of methyl groups. For the sets of molecules with one or two methyl groups, isomers with methyl groups bonded to P are more stable than those with methyl groups bonded to B, which is opposite the stabilities of the fluoro-substituted B P B P rings. If there is only one methyl group in a molecule or two methyl groups bonded to two different P or B atoms, bonding in an equatorial position is more favorable than an axial position. However, the most stable isomers with two methyl groups bonded to either P or B are the gem isomers, as is also the case for the fluoroderivatives. If the methyl groups are on two different P or B atoms, then the order of stability reflects the preference for bond formation in an equatorial position [(eq,eq) > (eq,ax) > (ax,ax)]. A similar pattern holds for molecules with four methyl groups: PMegem,PMegem > Mealleq > BMegem,BMegem > Meallax. The symmetries and relative energies of transition structures are also reported in Table 1. These transition structures present 10503
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Figure 1. Structures of molecules 3 (PMeax) and 8 (P1Meax,P3Meeq).
Table 1. Symmetries and Relative MP2/aug-cc-pVTZ Energies (kJ mol 1) for Equilibrium and Transition Structures of B2P2(CH3)nH8 n equilibrium structures formula
No.
description
transition structures
sym
Erel
sym
Erel
B2P2H8
1
parent
C2v
0.0
D2h
8.4
B2P2(CH3)1H7
2
PMeeq
Cs
0.0
Cs
9.9
3
PMeax
Cs
2.7
4 5
BMeeq BMeax
Cs Cs
7.5 12.0
Cs
18.0
B2P2(CH3)2H6
B2P2(CH3)2H6
B2P2(CH3)4H4
6
P1Megem
Cs
0.0
C2v
8.5
7
PMeeq,PMeeq
C2v
3.2
C2v
14.5
8
P1Meax,P3Meeq
Cs
5.6
C2h
14.1
9
PMeax,PMeax
C2v
9.7
10
B2Megem
Cs
15.9
C2v
22.9
11
BMeeq,BMeeq
C2v
17.7
C2v
29.7
12 13
B2Meeq,B4Meax BMeax,BMeax
Cs C2v
21.8 27.8
C2h
29.7
14
PMegem,PMegem
C2v
0.0
D2h
6.5
15
Mealleq
C2v
18.9
C2v
33.3
16
BMegem,BMegem
C2v
30.1
D2h
33.3
17
Meallax
C2v
31.6
the barriers for the interconversion of two isomers, or of axial and equatorial bonds in the same isomer. It should be noted that just as the energies of the equilibrium structures are given relative to the lowest-energy isomer, the energies of transition structures are also relative to the lowest energy isomer on the surface. This is of particular importance for isomers 11 and 13 and 15 and 17. The barrier to convert 11 with two equatorial bonds to 13 with two axial bonds is 12.0 kJ mol 1. However, the barrier for the reverse reaction, the conversion of 13 to 11, is only 1.9 kJ mol 1. This means that except at very low temperatures, only isomer 11 would be present. A similar situation exists for 15 and 17. In this case, the all axial isomer lies only 1.7 kJ mol 1 below the transition structure, so it is once again the all equatorial isomer that dominates. The barrier to interchanging equatorial and axial bonds in 16 BMegem,BMegem is only 3.2 kJ mol 1. Spin Spin Coupling Constants. There are two questions that should be addressed prior to examining spin spin coupling constants for the methyl-substituted B P B P rings. The first pertains to the feasibility of computing all of the coupling terms
for some of these rings, and whether the FC term alone can serve as a good approximation to the value of total J. The second concerns the ability of the EOM-CCSD/(qzp,qz2p) method to produce reasonable and reliable values of these coupling constants. To assist in answering the first question, the components of J for selected molecules are reported in Table S3. There are five coupling constants that will be examined in detail in this study, namely, 1J(B P), 2J(P P), 1J(B C), 1J(P C), and 3J(P C). Table S3 shows that the FC term dominates but underestimates 1 J(B P) by 1 2 Hz. The FC term usually underestimates 2 J(P P) by 1 2 Hz, although it overestimates this coupling constant by a similar amount for the two molecules that have both methyl groups in axial positions (9 and 13). These differences are not significant because 2J(P P) values span a range of 100 Hz in the methyl-substituted B P B P rings. The FC terms for both 1J(B C) and 3J(P C) are essentially equal to the corresponding total coupling constants. The only significant differences between the FC term and total J are found for 10504
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1
J(P C), for which the FC term underestimates 1J(P C) by 4 5 Hz, due primarily to the 5 Hz contribution of the SD term. However, differences between FC terms and between
Table 2. Computed EOM-CCSD/(qzp,qz2p) and Experimental Values of P C Coupling Constants (Hz) molecule
computed
P(CH3)3
5.0
13.6a
31.0
39.7a,b
1
45.4
53a
2
16.0 22.0
14a 22a
cyclic PH(CH2)2 J(P C)
phosphine
experimental
J(P C) J(P C)
3
a
Ref 24. b The experimental value is taken from the compound 1-phenylphosphirane in which the hydrogen bonded to P is replaced by a phenyl group.
Table 3. Coupling Constants 1FC(B P) (Hz) for Molecules 1 17 molecule 1 2
1
FC(B P)
molecule
62.0 (P1 B)
3
63.4 (P1 B) 51.3 (P3 B)
4
51.9 (P B2)
10
49.1 (P B2)
11 12
51.9 51.8 (P B2)
7 8 9
55.5 (P B4)
51.7 (P B4)
Table 4. Coupling Constants 2FC(P P) for Molecules 1 17 molecule
2
FC(P P)
molecule
2
FC(P P)
1
138.6
51.8 (P B2)
2
116.8
10
248.9
55.5 (P B4)
3
119.7
11
206.0
55.7 (P B4)
6
FC(B P)
55.8 54.6 (P3 B)
5
1
corresponding 1J(P C) values for these molecules are within 1 Hz. Because it is our purpose to assess the sensitivity of these coupling constants to axial versus equatorial versus geminal arrangements and to determine whether the magnitudes reflect the number of substituents, the differences between the FC terms and total J are not significant. Given these comparisons and the high cost of computing the SD terms for some of these molecules, the FC terms will be used to compare coupling constants for the methyl-substituted B P B P rings. In previous papers, we have addressed the second question, and have demonstrated that computed EOM-CCSD/(qzp,qz2p) coupling constants are in good agreement with experimental data, with few exceptions.18,21 23 We also addressed this question in some detail in our paper on the fluoro-substituted B P B P rings.3 Although no spin spin coupling constants have been measured experimentally for the molecules investigated in the present study, some experimental values of P C coupling constants are available,24 and these are compared with computed EOM-CCSD/(qzp,qz2p) values in Table 2. It is apparent from Table 2 that the experimental values of 1J(P C) are very sensitive to the bonding environment of P, and that the computed values reflect that sensitivity. This is important for the present study in which we examine changes in 1J(P C) due to changes in the number and positions of P C bonds. However, it
13
51.4
67.7 (P1 B)
14
63.5
4
172.7
12
235.9
50.8 (P3 B)
15
58.0
5
203.5
13
276.4
60.9 62.3 (P1 B)
16 17
48.8 55.7
6 7
101.0 97.7
14 15
86.9 160.1
57.7 (P3 B)
8
99.7
16
363.1
59.6
9
120.1
17
226.2
Figure 2. FC terms for one-bond P B coupling constants in molecules B2P2(CH3)nHn 8. 10505
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Figure 3. FC terms for two-bond P P coupling constants in molecules B2P2(CH3)nHn 8.
Table 5. Coupling Constants 1FC(P C) and 3FC(P C) for Molecules 2 17 molecule
1
FC(P C)
3
FC(P C)
2
28.2
48.6
3
35.7
3.9
6
33.7 ax 26.6 eq
3.6 46.8
7
26.7
46.5
8
35.5 ax
3.8
27.8 eq
44.3
9
32.3
14
30.2 ax
0.8
25.9 eq
41.0
22.5 29.2
50.1 3.7
15 17
2.0
is disappointing that the computed absolute values of 1J(P C) underestimate the experimental values by about 8 Hz. Part of this difference may be attributed to geometry differences, because the computed geometries are equilibrium structures that neglect zero-point vibrational motion found in the experimental ground-state structures. However, computed values of 2J(P C) and 3J(P C) are in very good agreement with experimental data. 1 FC(B P). Table 3 reports the one-bond B P coupling constants, and Figure 2 displays these values in a scattergram. For molecules with one or two methyl groups, 1FC(B P) increases relative to the parent molecule 1 if the methyl group is bonded to P but decreases if the methyl group is bonded to B. The largest value of this coupling constant is found for 6 with geminal P C bonds. If neither B nor P is bonded to a methyl group, there is little change in 1FC(B P) relative to 1. For molecules with four methyl groups, the isomer with the methyl groups bonded to the two P atoms has the greatest value of this coupling constant, while the isomer with the methyl groups
bonded to the two B atoms has the smallest value. 1FC(B P) for the molecules in which the methyl groups occupy all equatorial or all axial positions (15 and 17) are intermediate between the geminal molecules (14 and 16) and relatively close to the parent molecule 1. 1FC(B P) does not discriminate well between axial and equatorial bonds. There are some dramatic differences between B P coupling constants in the methyl and fluoro derivatives.3 The most striking is the much larger range of values, from 40 to 110 Hz for the fluoro derivatives, compared to 45 to 70 Hz for the methyl derivatives. The largest value of the B P coupling constant in the methylsubstituted rings is found for 6 (P1Megem), with both methyl groups bonded to P. In contrast, for the fluoro derivatives, the values of this coupling constant are near 100 Hz for the two molecules with geminal B F bonds, B2Fgem and BFgem,BFgem. 2 FC(P P). Table 4 presents the FC terms for two-bond P P coupling constants, which are always positive. Because the magnetogyric ratio of 31P is positive, the corresponding reduced coupling constants are also positive and, thus, in violation of the Dirac Vector Model that states that reduced two-bond coupling constants are negative.25 The scattergram shown in Figure 3 illustrates that 2FC(P P) decreases from 20 to 50 Hz relative to the parent molecule when methyl groups are bonded only to P and also decreases as the number of methyl groups increases. In contrast, 2FC(P P) increases relative to the parent molecule when methyl groups are bonded only to B. The change in this coupling constant is dramatic as the number of methyl groups increases, with 2FC(P P) increasing from 139 Hz for 1 to 363 Hz for 16, BMegem,BMegem. 2 FC(P P) for 15 (Mealleq) and 17 (Meallax) lie between the two extremes, with 2FC(P P) increasing by 20 and 90 Hz, respectively, relative to 1. Absolute values of the two-bond B B coupling constants 2FC(B B) are less than 1 Hz and provide no structural information. Significant differences between the values of two-bond P P coupling constants are found for the fluoro- and methyl-substituted derivatives. This coupling constant spans a range of about 1000 Hz 10506
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Figure 4. FC terms for one- and three-bond P C coupling constants in molecules B2P2(CH3)nHn 8.
for the fluoro derivatives, compared to 400 Hz for the methyl derivatives. Moreover, the largest value for the fluoro-substituted derivatives occurs in the isomer with four equatorial bonds. 1 FC(P C), 3FC(P C), and 1FC(B C). Table 5 reports values 1 of FC(P C) and 3FC(P C), and Figure 4 displays these in a scattergram. The smallest values of 1FC(P C) are found for P C coupling in molecules in which the methyl groups occupy equatorial positions, with values ranging from 22 (15) to 28 (2) Hz. When the methyl groups occupy axial positions, 1FC(P C) increases and varies from 29 (17) to 36 (3) Hz. This pattern is opposite that found for the fluoro-derivatives for which 1J(P F) values are larger for P F equatorial bonds. For both P C equatorial and axial coupling, 1FC(P C) tends to decrease as the number of methyl groups increases. The three-bond P C coupling constants exhibit dramatically different behavior. These coupling constants are significantly greater than the one-bond P C couplings when coupling involves a methyl group which occupies an equatorial position, with values ranging from 40 to 50 Hz. The largest value is found for 15, with all bonds equatorial. In contrast, 3 FC(P C) coupling constants for molecules in which the methyl groups occupy axial positions have absolute values less than 4 Hz. Thus, one-bond P C coupling constants are sensitive to both the number of methyl groups and their positions axial or equatorial, while the three-bond coupling constants are extremely sensitive to the bonding positions of the methyl groups. The sensitivity of 3FC(P C) can be related to the P B P C dihedral angle. For molecules in which the methyl group is in an equatorial position, this angle varies between 141 and 149°. For axial methyl groups, the values of this angle range from 91 to 102°, as illustrated in Figure 5. This behavior is analogous to the Karplus relationship for 3J(H H) coupling, for which smaller values are observed for systems in which dihedral angles approach 90°. Similar to the methyl-derivatives, the largest values of 3FC(P C) in the fluoro derivatives are found when the coupled F occupies an equatorial position. However, in contrast to the
methyl derivatives, values of 1J(P F) are significantly greater than values of 3J(P F). 1 FC(B C) values are sensitive to the axial or equatorial positions of the methyl groups, but not to the number of methyl groups present. Thus, 1FC(B C) values are 47.3 ( 0.5 Hz when coupling involves an axial methyl group, and 54.1 ( 1.4 Hz when the methyl group occupies an equatorial position. The threebond B C coupling constants do not exceed 3 Hz and, thus, provide no additional information. Coupling Constants for Transition Structures. Values of 1 FC(B P), 2FC(P P), 1FC(P C), 3FC(P C), and 1FC(B C) for transition structures are reported in Table 6, along with the corresponding equilibrium values. Values of 1FC(B P) for transition structures are essentially equal to or the average of the values in the equilibrium structures which are connected by the transition structure. A similar situation exists for the two other one-bond coupling constants 1FC(P C) and 1FC(B C). For these two couplings, the value in the transition structure differs from the average value of the coupling constants in the corresponding equilibrium structures by no more than 2.5 Hz. Of course, the distinction between axial and equatorial is lost in the transition structures. There are significant differences between the values of 2FC(P P) in corresponding equilibrium and transition structures. For the parent molecule 1, 2FC(P P) decreases by 79 Hz in the transition structure. For the methyl-substituted rings, 2FC(P P) for transition structures designated as 2f3, 4f5, 6, 7f9, 8, 10, 12, 14, and 16 increases by 25 to 50 Hz relative to the corresponding equilibrium structure, or relative to the larger value of the two equilibrium structures that are connected by the transition structure. Because P P coupling constants are highly sensitive to structure and bonding, the increase in 2FC(P P) may be attributed at least in part to the change in the geometry of the B P B P ring, which becomes planar in the transition structures. There are two transition structures, 11f13 and 15f17 for which 2FC(P P) is essentially equal to the value in the higherenergy equilibrium structure. As noted above, structures 13 and 10507
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Table 6. 1J(B P), 2J(P P), 1J(P C), 3J(P C), and 1J(B C) (Hz) for Equilibrium and Transition Structuresa structure
symmetries
eq
TS
1
C2v/D2h
56
60
2f3 4f5
Cs/Cs Cs/Cs
62,55;63,51 52,56;52,56
63,53 52,56
6
Cs/C2v
68,51
68,51
7f9
C2v/C2v
61;60
60
8
Cs/C2h
62,58
60
10
Cs/C2v
49,56
49,55
11f13
C2v/C2v
52;51
51
12
Cs/C2h
52,52
52
14 15f17
C2v/D2h C2v/C2v
64 58,56
63 56
16
C2v/D2h
49
48
1
FC(P B) (Hz)
2
FC(P P)
1
C2v/D2h
139
60
2f3
Cs/Cs
117;120
170
4f5
Cs/Cs
173;204
235
6
Cs/C2v
101
151
7f9
C2v/C2v
98;120
150
8 10
Cs/C2h Cs/C2v
100 249
148 286
11f13
C2v/C2v
206;276
277
12
Cs/C2h
236
272
14
C2v/D2h
87
113
15f17
C2v/C2v
160;226
229
16
C2v/D2h
363
369
2f3 6
Cs/Cs Cs/C2v
28;36 34,27
32 30
7f9
C2v/C2v
27;32
31
8
Cs/C2h
36,28
31
14
C2v/D2h
30,26
27
15f17
C2v/C2v
23;29
27
2f3
Cs/Cs
49;-4
6
Cs/C2v
47,-4
19
7f9 8
C2v/C2v Cs/C2h
47;-2 44,-4
14 15
14
C2v/D2h
41,-8
18
15f17
C2v/C2v
50;+4
16
1
FC(P C)
3
FC(P C) (Hz) 15
1
a
FC(B C)
4f5
Cs/Cs
56;47
50
10
Cs/C2v
47,54
50
11f13
C2v/C2v
56;47
49
12 15f17
Cs/C2h C2v/C2v
55,48 55;47
51 49
16
C2v/D2h
48,53
50
A comma between two numbers indicates that they are from the same isomer; a semicolon indicates that they are from two different isomers.
Figure 5. Scattergram showing the groupings of 3J(P C) values as a function of the P B P C dihedral angle.
17 lie only 1.9 and 1.7 kJ mol 1, respectively, below the corresponding transition structures, with the result that these structures and the corresponding transition structures have very similar geometries. For the same reason, 2FC(P P) for 16 and its transition structure have essentially the same values of 2FC(P P), because the barrier for interchanging equatorial and axial bonds is only 3.2 kJ mol 1. The only other coupling constant for which the values in the transition structures and corresponding equilibrium structures are significantly different is 3FC(P C). For all transition structures, 3FC(P C) values lie between 14 and 19 Hz, independent of the number of methyl groups. These values are intermediate between the values of 3FC(P C) for coupling involving equatorial and axial bonds. This difference may also be related to the values of the P B P C dihedral angles in transition structures which lie between 115 and 120° and are thus intermediate between the values for axial and equatorial bonds. This relationship can readily be seen in Figure 5.
’ CONCLUSIONS Ab initio calculations have been performed on a series of methyl-substituted four-member B P B P rings with the formula B2P2(CH3)nH8 n, for n = 0, 1, 2, 4. The results of these calculations support the following statements. 1 All molecules B2P2(CH3)nH8 n have puckered B P B P rings with axial and equatorial P CH3 and B CH3 bonds. 2 Isomers with the CH3 group bonded to P are more stable than those with CH3 bonded to B. If only one methyl group is present or if two methyl groups are bonded to two different P or B atoms, isomers with equatorial P C or B C bonds are more stable than those with axial bonds, although the gem isomer is the most stable B2P2(CH3)2H6 isomer among those with two P C or two B C bonds, respectively. For molecules B2P2(CH3)4H4, the order of stabilities reflects the overall trend: PMegem,PMegem > Mealleq > BMegem,BMegem > Meallax. 3 Transition structures for the interconversion of a pair of isomers or the exchange of axial and equatorial positions in the same isomer have been obtained. The barriers for converting isomers with two axial B C bonds and with four axial bonds to the corresponding isomers with 10508
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equatorial bonds are very low, to the extent that only the isomers with equatorial bonds would exist, except at very low temperatures. A low barrier also exists for interchanging axial and equatorial positions in the isomer with geminal B C bonds at both B atoms. The coupling constant 1FC(B P) in methyl-substituted B P B P rings increases relative to the unsubstituted parent molecule if the methyl group is bonded to P, but decreases if the methyl is bonded to B. In contrast, 2J(P P) decreases if methyl groups are bonded only to P but increases if they are bonded to B. 1 J(P C) is sensitive to the axial or equatorial positions of methyl groups but 1J(B C) is not. 3J(P C) is significantly greater than 1J(P C) for molecules in which the methyl group occupies an equatorial position. Values of 3J(P C) for axial and equatorial P C bonds and P C bonds in transition structures are clearly dependent on the values of the P B P C dihedral angle. 1J(B C) is sensitive to the axial or equatorial positions of B C bonds. 1 FC(B P), 1FC(P C), and 1FC(B C) for a transition structure are essentially equal to, or the average of the values for the equilibrium structures which are connected by the transition structure. Values of 2FC(P P) increase significantly relative to the larger of the values for the corresponding equilibrium structures with three exceptions. For two of these, the transition structure and the higher-energy equilibrium structure have essentially equal values of 2FC(P P), because the higher-energy structure is structurally and energetically similar to the transition structure. For the third case, the barrier to interconverting B C geminal bonds in B2P2(CH3)4H4 is very low. 3FC(P C) values for transition structures are essentially independent of the number of methyl groups present. The values of 3FC(P C) and the associated P B P C dihedral angles are intermediate between values for methyl groups in equatorial and axial positions. Coupling constants in the methyl and the corresponding fluoro derivatives of the 1,3-diborata-2,4-diphosphoniocyclobutanes demonstrate the great sensitivity of phosphorus coupling to structural and electronic effects.
’ ASSOCIATED CONTENT
bS Supporting Information. MP2/aug-cc-pVTZ equilibrium and transition structures and total energies; PSO, DSO, FC, and SD components of coupling constants. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected].
’ ACKNOWLEDGMENT Thanks are due to the Ohio Supercomputer Center for continuing support of this research. We also thank the Ministerio de Ciencia e Innovacion (Project No. CTQ2009-13129-C02-02) and the Comunidad Autonoma de Madrid (Project MADRISOLAR2, ref S2009/PPQ-1533) for continuing support.
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