Article pubs.acs.org/JPCA
Exploring the (H2CPH2)+:N-Base Potential Surfaces: Complexes Stabilized by Pnicogen, Hydrogen, and Tetrel Bonds Janet E. Del Bene,*,‡ Ibon Alkorta,*,§ and José Elguero§ ‡
Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555, United States Instituto de Química Médica (IQM-CSIC), Juan de la Cierva, 3, E-28006 Madrid, Spain
§
S Supporting Information *
ABSTRACT: Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to determine the structures, binding energies, and bonding properties of complexes involving the cation (H2CPH2)+ and a set of sp-hybridized nitrogen bases including NCCH3, NP, NCCl, NCH, NCF, NCCN, and N2. On each (H2CPH2)+:N-base surface, four types of unique equilibrium structures exist: a complex with a P···N pnicogen bond formed through the π system of (H2CPH2)+ (ZB-π); a complex with a P···N pnicogen bond formed through the σ system of (H2CPH2)+ (ZBσ); a hydrogen-bonded complex with a PH···N hydrogen bond (HB); and a tetrel-bonded complex with a C···N bond (TB). Binding energies of complexes stabilized by the same type of intermolecular interaction decrease in the order NCCH3 > NP > NCCl > NCH > NCF > NCCN > N2. For a given base, binding energies decrease in the order ZB-π > HB > ZB-σ > TB, except for a reversal of HB and ZB-σ with the weakest base N2. Binding energies of ZB-π, HB, and ZB-σ complexes increase exponentially as the corresponding PN distance decreases, but the correlation is not as good between the binding energies of TB complexes and the intermolecular CN distance. Chargetransfer energies stabilize all complexes and also exhibit an exponential dependence on the corresponding intermolecular distances. EOM-CCSD spin−spin coupling constants 1pJ(PN) for ZB-π and ZB-σ complexes, and 2hJ(PN) for HB complexes increase quadratically as the corresponding PN distance decreases. Values of 1tJ(CN) for TB are small and show little dependence on the CN distance. 1J(PH) values for the hydrogen-bonded PH bond in HB complexes correlate with the corresponding PH distance, whereas values of 1J(PH) for the non-hydrogen-bonded PH correlate with the PN distance.
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INTRODUCTION Noncovalent interactions are important both in condensed media and often in the gas phase. Although the hydrogen bond is the most important noncovalent intermolecular interaction, other interactions such as the pnicogen bond1 and the tetrel bond2 have come to the forefront in the recent literature. These bonds arise from Lewis acid−Lewis base interactions in which the Lewis acid corresponds to an atom from the pnicogen (N, P, As, and Sb), or the tetrel (C, Si, Ge, and Sn) families. Formation of both the pnicogen bond and the tetrel bond can be described in terms of σ-holes, as suggested by Politzer and Murray.3 Although pnicogen bonds have been known for some time, there has been a resurgence of interest in this bond beginning with two papers published in 2011.1,4 The majority of recent studies have focused primarily on pnicogen bonds in neutral complexes,5−7 but there have also been a few studies of pnicogen-bonded anionic8−10 and cationic11−14 complexes. We recently described systematic studies of the P···N pnicogen bond in F4−nHnP+:N-base complexes with approximately linear arrangements of FP···N and HP···N across the pnicogen bond.15−17 © XXXX American Chemical Society
Tetrel-bonded clusters especially for complexes with Si and Ge18−22 have also been known for some time, but interest in this noncovalent interaction has grown with the publication of two recent articles that coined the terms tetrel bond2 and carbon bond.23 A recent computational study has associated the tetrel bond with the intermediate in an SN2 reaction.24 As a continuation of our studies of intermolecular interactions, we recently explored the potential energy surfaces that arise from the interaction of the cation H2CPH2+ with the sp-hybridized bases NCCH3, NP, NCCl, NCH, NCF, NCCN, and N2, in search of stable complexes H2CPH2+:Nbase. In this paper we present and discuss the structures and binding energies of these complexes, their bonding properties, charge-transfer interactions, and nuclear spin−spin coupling constants involving atoms that participate in the various intermolecular interactions. Received: July 15, 2015 Revised: August 24, 2015
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METHODS The structures of the isolated monomers and the complexes (H2CPH2)+:N-base for the bases NCCH3, NP, NCl, NCH, NCN, NCCN, and N2 were optimized at second-order Møller−Plesset perturbation theory (MP2)25−28 with the aug′-cc-pVTZ basis set.29 This basis set is derived from the Dunning aug-cc-pVTZ basis set30,31 by removing diffuse functions from hydrogen atoms. Frequencies were computed to establish that the optimized structures correspond to equilibrium structures on their potential surfaces. Complex binding energies are reported as the negative energy (−ΔE) for the reaction (H 2CPH 2)+ + N‐base → (H 2CPH 2)+ :N‐base
Figure 1. MEP on the 0.001 au electron density isosurface of (H2C PH2)+. The color code indicates decreasing values blue > green > yellow > red. The locations of the σ-holes are indicated with black dots, and their values are given in au. The 0.193 au holes are located above and below the symmetry plane containing the ion.
All calculations were performed using the Gaussian 09 program.32 The MP2 electron densities of the complexes have been analyzed using the atoms in molecules (AIM) methodology,33−36 employing the AIMAll37 program. The topological analysis of the electron density produces the molecular graph of each complex. This graph identifies the location of electron density features of interest, including the electron density (ρ) maxima associated with the various nuclei, saddle points that correspond to bond critical points (BCPs), and ring critical points that indicate a minimum electron density within a ring. The zero gradient line that connects a BCP with two nuclei is the bond path. The electron density at the BCP (ρBCP) and the total energy density (HBCP) are additional useful quantities for characterizing interactions.38 In addition, the natural bond orbital (NBO)39 method has been used to analyze the stabilizing charge-transfer interactions employing the NBO-6 program.40 Because MP2 orbitals are nonexistent, the chargetransfer interactions have been computed using the B3LYP functional41,42 with the aug′-cc-pVTZ basis set at the MP2/ aug′-cc-pVTZ complex geometries, so that at least some electron correlations effects could be included. Spin−spin coupling constants were evaluated using the equation-of-motion coupled cluster singles and doubles (EOMCCSD) method in the CI (configuration interaction)-like approximation,43,44 with all electrons correlated. For these calculations, the Ahlrichs45 qzp basis set was placed on 13C, 15 N, and 19F, and the qz2p basis set on 31P, 35Cl, and the hydrogen-bonded 1H atom. The Dunning cc-pVDZ basis was placed on all other 1H atoms. The EOM-CCSD calculations were performed using ACES II46 on the IBM Cluster 1350 (Glenn) at the Ohio Supercomputer Center.
Descriptions of Complexes Found on the Intermolecular Surfaces. Four unique types of complexes have been found on each intermolecular surface. Two are pnicogenbonded at P that are designated as ZB-π and ZB-σ, one is a hydrogen-bonded complex with a PH···N hydrogen bond (HB), and one is a tetrel-bonded complex at C (TB). These are illustrated in Figure 2 by (H2CPH2)+:NCH. The structures, total energies, and molecular graphs of all complexes are given in Table S1. The H2CPH2+ ion has two symmetry planes, the first the plane of the ion and the second a plane perpendicular to the plane of the ion. Although the σ-hole at P is found in the plane of the ion between the two PH bonds, the corresponding pnicogen-bonded complex ZB-π forms in the second symmetry plane. From Figure 2 it can be seen that in the (H2C PH2)+:NCH ZB-π complex, the ion H2CPH2+ loses planarity as the two PH bonds bend away from the approaching base, and the pnicogen bond forms in the second symmetry plane. The angle CPN in these complexes is 126 or 127°, except for (H2CPH2)+:N2, which has an angle of 133°. The P NC angle varies between 167 and 175°. The complex HB is a hydrogen-bonded complex in which the nitrogen base donates a pair of electrons to the ion (H2C PH2)+ through the σ-hole on the extension of a PH bond, to form a PH···N hydrogen bond. The hydrogen bonds in all of these complexes are linear, as is usually the case for cationic hydrogen bonds. The complex ZB-σ is a pnicogen-bonded complex that forms in the plane of the ion through the σ-hole at P located in the region of the PC bond. The NPH angle has a value of either 163 or 164°, whereas the PNC angle lies between 175 and 177°. Thus, the arrangement of atoms across the pnicogen bond approaches linearity, as is typical for pnicogenbonded complexes. Finally, a tetrel-bonded complex designated TB forms above the plane of the ion in the second symmetry plane through a πhole at C. In these complexes, the PCN angle is between 118 and 125°, whereas the CNC angle approaches linearity, with values between 174 and 180°. Structures and Binding Energies of Complexes. The binding energies of (H2CPH2)+:N-base complexes are reported in Table 1. For a fixed type of complex, binding energies decrease as the base strength decreases in the order NCCH3 > NP > NCCl > NCH > NCF > NCCN > N2. For a
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RESULTS AND DISCUSSION Monomers. The structure of the isolated cation (H2C PH2)+ has C2v symmetry and is reported in Table S1 of the Supporting Information. The molecular electrostatic potential (MEP) on the 0.001 au electron density isosurface exhibits σholes in the symmetry plane containing the ion along the extensions of the PH bonds, between the two PH bonds, and in the region of the PC bond. In addition, there are πholes near C that are located above and below this symmetry plane, in the symmetry plane perpendicular to the plane of the ion. These holes are shown in Figure 1 and will be related to the structures of the various (H2CPH2)+:N-base complexes. No σ-holes exist along the extensions of the CH bonds, and no equilibrium structures with CH···N hydrogen bonds have been found on the intermolecular surfaces. B
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Figure 2. (H2CPH2)+:NCH complexes ZB-π, HB, ZB-σ, and TB.
Table 1. Binding Energies (−ΔE, kJ·mol−1) of Complexes (H2CPH2)+:N-base type/base
NCCH3
NP
NCCl
NCH
NCF
NCCN
N2
ZB-π HB ZB-σ TB
107.6 88.2 83.7 63.6
103.4 78.8 71.2 54.6
85.0 70.3 68.4 51.0
78.9 67.4 65.6 49.2
70.8 60.5 59.5 43.9
48.2 40.5 39.7 27.6
17.7 15.1 15.6 10.5
above, they do not reflect the relative stabilities of the HB and ZB-σ complexes. Table 2 reports the intermolecular PN distances in the pnicogen-bonded complexes ZB-π and ZB-σ, and the hydrogen-bonded complexes HB. For each base, the PN distance decreases in the order HB > ZB-σ > ZB-π. Figure 3 illustrates that the binding energies of each type of complex increase exponentially as the PN distance decreases, with correlation coefficients of 0.964, 0.953, and 0.991, respectively. It is apparent from Figure 3 that the binding energies of the more strongly bound ZB-σ and HB complexes are comparable to the binding energies of some of the ZB-π complexes, even though the PN distances in ZB-σ and HB are longer. Table 2 also reports the CN distances in the tetrel-bonded complexes TB. The binding energies of TB complexes also increase exponentially as the CN distance decreases, but with a reduced correlation coefficient of 0.886. AIM and NBO Results. The molecular graphs obtained from the AIM analyses carried out on the complexes ZB-π, HB, ZB-σ, and TB are given in Table S1 of the Supporting Information. The molecular graphs of the ZB-σ complexes have two bond paths that suggest the interactions that stabilize these complexes are PH···N and CH···N hydrogen bonds. However, although the H···N interactions are most probably stabilizing electrostatic interactions, they do not correspond to hydrogen bonding interactions, because the hydrogen bonds are very nonlinear, with HPN and HCN angles around 45°. For XH···Y hydrogen bonds, values of the H XY angle greater than about 30° correspond to very weak or nonexistent hydrogen bonds. The paths to the H atoms most probably reflect the diffuseness of the σ-hole in the CP region. Moreover, the nature of the charge-transfer interactions is consistent with the existence of pnicogen-bonded complexes, as discussed in greater detail below. Table S2 of the Supporting Information presents values of the electron density at bond critical points (ρBCP) and the energy density at those points (HBCP). The Laplacians have also
Table 2. Intermolecular PN and CN Distances (R, Å) of Complexes (H2CPH2)+:N-basea type/ base
NCCH3
NP
NCCl
NCH
NCF
NCCN
N2
ZB-π HB ZB-σ TB
2.263 3.163 3.051 2.652
2.193 3.122 3.083 2.561
2.375 3.241 3.089 2.722
2.480 3.281 3.123 2.763
2.519 3.297 3.126 2.776
2.626 3.374 3.193 2.859
3.068 3.646 3.414 3.094
a Distances are PN distances, except for TB complexes which are CN distances.
given base, binding energies decrease with respect to the nature of the interaction in the order ZB-π > HB > ZB-σ > TB, except for a reversal of HB and ZB-σ when N2 is the base. This order does not correspond to the relative values of the σ-holes. The binding energies of the pnicogen-bonded ZB-π complexes vary between 18 kJ·mol−1 for the complex with N2 and 108 kJ·mol−1 for that with NCCH3, whereas the binding energies of the tetrel-bonded complexes range between 11 and 64 kJ·mol−1 for complexes with these same two bases. The hydrogen-bonded HB and pnicogen-bonded ZB-σ complexes with a given base have binding energies that are within 2 kJ·mol−1, except for the complexes with NCCH3 and NP. With these two bases, the HB complexes are 4.5 and 7.6 kJ· mol−1, respectively, more stable than the ZB-σ complexes. The values of the σ-holes at P along the CP bond and along the extension of the PH bonds are intermediate between those associated with the ZB-π and TB complexes, but as noted C
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Figure 3. Binding energies versus the intermolecular PN distance for complexes ZB-π, HB, and ZB-σ.
Figure 4. Negative of the energy density at the BCP versus the intermolecular PN distance for ZB-π and HB complexes.
been computed and are all positive, but they are not reported because their interpretation in complexes with P···N and P···O bonds is ambiguous, as noted previously.47,48 In contrast, the electron densities at bond critical points increase exponentially with decreasing corresponding distances, with correlation coefficients of 0.995 or greater. Negative energy densities are indicative of intermolecular bonds with some covalent character. These densities for the ZB-π complexes are negative and increase exponentially as the PN distance decreases. The trendline shown in Figure 4 has a correlation coefficient of 0.902. The energy densities are negative for six of the HB complexes and exhibit an exponential
dependence on the intermolecular PN distance as illustrated in Figure 4, with a correlation coefficient of 0.958. The one exception is the most weakly bound (H 2 CPH 2 ) + :N 2 complex. In contrast, energy densities are positive for the more weakly bound ZB-σ and TB complexes, with values between 0.001 and 0.002 au. The stabilizing charge-transfer energies obtained from the NBO analyses are reported in Table 3. Charge-transfer energies span a very large range, from 4 kJ·mol−1 in the TB complex (H2CPH2)+:N2 to 178 kJ·mol−1 in the HB complex (H2C PH2)+:NP. For a given base, the order of decreasing chargetransfer energy is HB > ZB-π ≫ TB > ZB-σ, which is not the D
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The Journal of Physical Chemistry A Table 3. Total Charge-Transfer Energies (kJ·mol−1) for (H2CPH2)+:N-base Complexes ZB-π, HB, ZB-σ, and TB base NCCH3 NP NCCl NCH NCF NCCN N2
ZB-π
HB
ZB-σ
Nlp→σ*PC
Nlp→σ*PH
Nlp→σ*PH
123.3 127.2 87.0 63.1 53.9 37.2 9.0
162.3 177.5 109.8 94.8 85.7 62.8 22.2
10.5 10.0 8.9 8.0 8.0 6.3 3.3
(6.4) (5.2) (5.4) (4.9) (4.8) (3.8) (1.9)
Table 4. Coupling Constants 1pJ(PN) for ZB-π and ZB-σ, 2h J(PN) for HB, and 1tJ(CN) for TB Complexes (H2CPH2)+:N-base
TB a
Nlp→σ*CP
ZB-π base
24.6 23.2 19.3 17.5 16.1 12.5 4.0
NCCH3 NP NCCl NCH NCF NCCN N2
a
Values in parentheses refer to the dominant charge-transfer interaction from the nitrogen lone pair to the PHt bond, which approaches a linear NPHt alignment. The remaining part of the total charge-transfer energy involves transfer from the N lone pair to the PHc bond that is closer to N.
1p
J(PN) 113.4 124.3 81.5 54.3 46.5 25.8 −3.0
HB 2h
J(PN)
−123.8 −131.4 −95.7 −81.6 −80.5 −59.0 −19.6
ZB-σ 1p
J(PN) −27.3 −26.3 −24.2 −21.1 −22.0 −17.0 −7.0
TB 1t
J(CN) −4.7 −4.3 −4.4 −4.1 −4.2 −3.6 −1.8
usually increase with decreasing distance, the greater Nlp→ σ*PHt charge-transfer energies are another indication that these complexes are pnicogen-bonded rather than hydrogenbonded. Spin−Spin Coupling Constants. The paramagnetic spin− orbit (PSO), diamagnetic spin−orbit (DSO), Fermi contact (FC), and spin-dipole (SD) components of coupling constants 1p J(PN) for complexes ZB-π and ZB-σ, 2hJ(PN), 1hJ(H N), and 1J(PH) for HB complexes, and 1tJ(NC) for TB complexes are reported in Table S3 of the Supporting Information. These data illustrate that the dominant FC terms provide excellent approximations to the total coupling constants across pnicogen, hydrogen, and tetrel bonds, as well as 1hJ(HP) and 1J(PH) for HB complexes. Coupling across Intermolecular Bonds. Table 4 provides the total coupling constants across the intermolecular bonds for all complexes. 1pJ(PN) has a value of −3 Hz for the (H2C PH2)+:N2 ZB-π complex and increases to 124 Hz in (H2C PH2)+:NP. As illustrated in Figure 6, this coupling constant
same as the order of decreasing binding energies. For each type of interaction, the charge-transfer energy decreases as the strength of the base decreases, except for a reversal of NP and NCCH3 in ZB-π and HB complexes. Figure 5 illustrates excellent exponential correlations between increasing chargetransfer energy and decreasing intermolecular distance. The correlation coefficients of the trendlines are 0.993, 0.997, 0.990, and 0.923 for the complexes ZB-π, HB, ZB-σ, and TB, respectively. The total charge-transfer energies reported in Table 3 for ZB-σ complexes are sums of two charge-transfer interactions, Nlp→σ*PHt and Nlp→σ*PHc, with PHt and PHc, the PH bonds that are trans and cis, respectively, to N with respect to the PC bond. For all complexes, Nlp→σ*PHt is greater than Nlp→σ*PHc, even though the NHc distance is much shorter than NHt. Because charge-transfer energies
Figure 5. Charge-transfer energies versus the intermolecular PN distance for ZB-π, HB, and ZB-σ complexes, and the CN distance for TB complexes. E
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Figure 6. 1pJ(PN) versus the PN distance for ZB-π complexes (H2CPH2)+:N-base.
Figure 7. 2hJ(PN) and 1pJ(PN) versus the PN distance for complexes HB and ZB-σ, respectively, and 1tJ(CN) versus the CN distance for TB complexes (H2CPH2)+:N-base.
PH2)+:NP. The second-order dependence of these coupling constants on the corresponding intermolecular distance is illustrated in Figure 7. The correlation coefficients of the trendlines are 0.996 and 0.990 for HB and ZB-σ complexes, respectively. 1tJ(CN) shows little dependence on the PN distance, varying between −2 and −5 Hz. 1 J(PH). Table 5 reports the PH distances and values of 1 J(PH) for the hydrogen-bonded and non-hydrogen-bonded
exhibits a second-order dependence on the PN distance, with a correlation coefficient of 0.994. In contrast to 1pJ(PN) for the ZB-π complexes, 1pJ(PN) is negative for coupling across the pnicogen bond in ZB-σ complexes, varying from −7 Hz in (H2CPH2)+:N2 to −27 Hz in (H2CPH2)+:NCCH3. The coupling constant 2hJ(P N) across the hydrogen bond is also negative and varies between −20 Hz in (H2CPH2)+:N2 and −131 Hz in (H2C F
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The Journal of Physical Chemistry A Table 5. PH Distances (Å) and 1J(PH) Values (Hz) for Hydrogen-Bonded and Non-Hydrogen-Bonded PH Bonds in HB Complexes hydrogen-bonded base
R(PH)
NCCH3 NP NCCl NCH NCF NCCN N2 Ion
1.454 1.478 1.431 1.426 1.420 1.413 1.397 1.391
1
Figure 8. Although the non-hydrogen-bonded PH bond distance changes very little upon complexation, increasing from 1.391 Å in the ion to only 1.395 Å in (H2CPH2)+:NP, 1 J(PH) decreases dramatically to 612 Hz in (H2C PH2)+:N2 and then to 482 Hz in (H2CPH2)+:NP. How can these results be explained? The order of decreasing 1J(PH) for the non-hydrogenbonded PH in complexes (H2CPH2)+:N-base with respect to the base is the same as the order of decreasing PN distance in these complexes. This suggests that the presence of the hydrogen bond significantly changes the electron densities of the P and/or non-hydrogen-bonded H both in the ground state and in the excited states that couple to it through the FC operator. As a result, 1J(PH) for the non-hydrogen-bonded PH bond varies upon complex formation more than 1J(P H) for the hydrogen-bonded PH. Figure 9 provides a plot of 1 J(PH) for the non-hydrogen-bonded PH versus the intermolecular PN distance. The value of this coupling constant increases quadratically as the PN distance increases, with a correlation coefficient of 0.997. This behavior is consistent with the value of 637 Hz for 1J(PH) in the isolated ion, that is, the value at infinite PN distance. In contrast, 1J(PH) for the hydrogen-bonded PH shows no correlation with the PN distance, but as noted above, it does correlate with the corresponding PH distance. Further insight into the coupling behavior of hydrogenbonded atoms may be obtained from Figure 10 in which the third hydrogen-bonding coupling constant 1hJ(HN) is plotted against the HN distance. This plot indicates that 1h J(HN) increases as the HN distance decreases but then begins to decrease at short HN distances. If proton transfer were to occur in the complex with the shortest PN distance, 1h J(HN) for the complex (H2CPH2)+:NP would become 1 J(HN) for the cation +HNP. The value of this coupling
non-hydrogen-bonded
J(PH)
R(PH)
611.9 583.7 623.2 624.4 624.4 619.2 600.5 636.6
1.393 1.395 1.392 1.392 1.392 1.392 1.391 1.391
1
J(PH) 503.7 481.8 536.9 546.0 555.1 572.6 611.8 636.6
PH bonds in HB complexes. 1J(PH) for the isolated cation (H2CPH2)+ is 637 Hz at a PH distance of 1.391 Å. The hydrogen-bonded PH distance increases upon complex formation to 1.397 Å in (H2CPH2)+:N2 and has its largest value of 1.478 Å in (H2CPH2)+:NP. Figure 8 shows that 1 J(PH) decreases relative to the cation to 601 Hz in the complex with N2, then increases with increasing PH distance, and finally decreases once again as the PH distance continues to increase. Decreasing 1J(PH) with increasing PH distance is consistent with the expectation that at very long PH distances, proton transfer could occur and there would no longer be a PH covalent bond. The correlation coefficient of the second-order trendline relating 1J(PH) to the PH distance in Figure 8 is 0.986. To gain further insight into the variation of the value of the hydrogen-bonded 1J(PH) coupling constant, it is advantageous to examine the values of 1J(PH) for the non-hydrogenbonded PH bond. These values are also reported in Table 5 and are plotted against the corresponding PH distance in
Figure 8. 1J(PH) versus the corresponding PH distance for hydrogen-bonded and non-hydrogen-bonded PH bonds in HB complexes (H2CPH2)+:N-base. G
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Figure 9. 1J(PH) versus the PN distance for the non-hydrogen-bonded PH in HB complexes (H2CPH2)+:N-base.
Figure 10. 1hJ(HN) versus the HN distance for HB complexes (H2CPH2)+:N-base.
constant in the isolated +HNP ion is −120 Hz at an HN distance of 1.017 Å. Although the HN distance and 1hJ(H N) in the complex (H2CPH2)+:NP are far from the corresponding values for a covalent HN bond in +HNP, it is reasonable to suggest that 1hJ(HN) would become negative if the HN distance decreased sufficiently.
nitrogen bases NCCH3, NP, NCCl, NCH, NCF, NCCN, and N2. The following statements are supported by the results obtained in this study. 1. On each (H2CPH2)+:N-base surface, four unique types of equilibrium structures have been found. These include a pnicogen-bonded complex through the π system of (H2CPH2)+ (ZB-π), a pnicogen-bonded complex through the σ system of (H2CPH2)+ (ZB-σ), a hydrogen-bonded complex with a PH···N hydrogen
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CONCLUSIONS Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate the complexes (H2CPH2)+:N-base for the H
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2.
3.
4. 5.
6.
7.
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Thanks are also given to the Ohio Supercomputer Center and CTI (CSIC) for their continued computational support.
bond (HB), and a tetrel-bonded complex with a C···N bond (TB). For each type of complex, binding energies decrease in the order NCCH3 > NP > NCCl > NCH > NCF > NCCN > N2. For a given base, binding energies decrease in the order ZB-π > HB > ZB-σ > TB, except for a reversal of HB and ZB-σ when N2 is the base. Binding energies of ZB-π, HB, and ZB-σ complexes increase exponentially as the corresponding intermolecular PN distances decrease. Binding energies of TB complexes correlate with the CN distance, but with a reduced correlation coefficient. σ- and π-holes are very useful for identifying acidic sites for complex formation, but their values are not reliable predictors of relative binding energies. Charge-transfer energies stabilize all complexes and also increase exponentially as the corresponding intermolecular distances decrease. The nature of the charge-transfer interaction is consistent with the nature of the intermolecular bond. Spin−spin coupling constants 1pJ(PN) for ZB-π and ZB-σ complexes, and 2hJ(PN) for HB complexes exhibit second-order correlations with the corresponding PN distances. Values of 1tJ(CN) are small and show little dependence on the CN distance. 1 J(PH) for the hydrogen-bonded and the nonhydrogen-bonded PH bonds exhibit very different distance dependencies. Relative to isolated (H2C PH2)+, 1J(PH) for the hydrogen-bonded PH bond initially decreases, then increases and decreases again as the PH distance elongates. In contrast, the nonhydrogen-bonded PH distance changes very little upon complexation, but 1J(PH) decreases dramatically relative to the isolated cation, and correlates with the P N distance. 1hJ(HN) correlates with the HN distance.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b06828. Geometries, total energies, and molecular graphs of (H2CPH2)+ and complexes (H2CPH2)+:N-base; values of electron densities and energy densities at P··· N bond critical points; PSO, DSO, FC, and SD components of coupling constants; full refs 32 and 46 (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*J.E.D.B. E-mail:
[email protected]. Phone: +1 330-609-5593. *I.A. E-mail:
[email protected]. Phone: +34 915622900. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was carried out with financial support from the Ministerio de Economiá y Competitividad (Project No. CTQ2012-35513-C02-02) and Comunidad Autónoma de Madrid (Project FOTOCARBON, ref S2013/MIT-2841). I
DOI: 10.1021/acs.jpca.5b06828 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
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