Dipoles Inside of Dipoles: Insertion Complexes of Polar versus

Jun 5, 2017 - Faculty of Science, University of Ontario Institute of Technology (UOIT), Oshawa, Ontario L1H 7K4, Canada. J. Phys. Chem. A , 2017, 121 ...
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Dipoles Inside of Dipoles: Insertion Complexes of Polar versus Nonpolar Molecules in Ion Pairs Fedor Y. Naumkin* Faculty of Science, University of Ontario Institute of Technology (UOIT), Oshawa, Ontario L1H 7K4, Canada S Supporting Information *

ABSTRACT: Highly polar molecular systems are in demand as a means of enabling many important practical applications based on light−matter interactions. In the present work, the insertion complexes of recently synthesized polar molecules trapped between alkali halide counterions are studied. For specific selected compositions, the M−molecule−X systems are predicted to be stable to dissociation into molecule + alkali halide. It is found that unlike their nonpolar molecule-based counterparts, the polar molecule complexes can be even more stable than their common dipole−dipole MX−molecule isomers. This makes them thermodynamically stable, highly polar species, with very large dipoles of about 20 D, and they could be used, for example, to develop efficient light sensors. Furthermore, due to the neutralization of the M−X charge transfer in the excited triplet state, such complexes represent unique spin-controlled dipole-switch molecular systems with the large dipole turned off and even inverted by the spin state for the nonpolar and polar molecule complexes, respectively. This potentially could allow various spintronic and optoelectronic applications. In addition, the IR intensity spectra are predicted to sensitively indicate the formation of both the M−molecule−X and MX−molecule isomers, thus facilitating their reliable detection and differentiation in experiments. Exclusion has, however, occurred for the M−C2F6−F systems, with the framing F atom forming a covalent bond with the (effectively pentavalent) C atom, leading to a considerable charge on the trapped molecule.9,10 Other cases have involved unsaturated molecules such as benzene and its hexafluoro derivative.11,12 The above systems are typically metastable, that is, higher in energy relative to mol + MX, due to separation of the counterions and have relatively low potential barriers (up to about 0.2 eV for M−C4H8−X) to their recombination. This, in principle, could enable efficient energy storage at the molecular level, which would require low temperatures to stabilize the systems. In the present work, mol = C6H12 and its polar analogue allcis C6H6F6 are involved in order to investigate two possible means of further stabilization of such systems: via a larger area of the trapped molecule (needing longer M−X stretches to go around it) and stronger electrostatic interactions between the ions and the molecule. According to the earlier studies for mol = C3H6 and C4H8, the MX = CsCl and CsI cases correspond to more stable M−mol−X species (apparently due to a smaller perturbation of longer MX by insertion), the Cs−C3H6−I being even slightly more stable relative to CsI + C3H6. Therefore, the present work focuses on these alkali halides.

1. INTRODUCTION Molecules with large dipole moments are of interest for improving the efficiency of lasers, solar cells, and other electronic devices.1,2 Optoelectronics is another area of possible applications of such molecules strongly interacting with light. A recently produced covalently bonded diaminobenzenetetracarbonitrile has a record dipole of about 14 D,3 which exceeds even the values for ionic systems such as alkali halides (up to about 12 D for CsI). Another recent champion is the allcis C6H6F6 molecule (about 6 D).4 In fact, the present work computationally combines the latter two species as components of novel insertion complexes, with the molecule trapped between the counterions. When a molecule is inserted between the oppositely charged ions, for example, of alkali halide MX, they are pushed apart, which increases considerably the dipole moment of the M− mol−X system. A suitable geometry of the molecule can provide a concave electron density holding the counterions and preventing their recombination. Appropriately shaped saturated hydrocarbons (and their halo derivatives) represent one class of such ion separators interacting with the ions noncovalently. Previous studies have included a series of M−mol−X systems, with mol being hexafluoroethane (while ethane fails to prevent the ion recombination), cyclopropane, cyclobutane, or cyclohexane.5−8 In these complexes, the nonpolar molecules remain neutral or only weakly charged while being polarized in the electric field of the framing ions, which could even reshape the molecule by compression due to their mutual attraction.7 © XXXX American Chemical Society

Received: March 18, 2017 Revised: May 21, 2017

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DOI: 10.1021/acs.jpca.7b02576 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Recent directly relevant work has addressed, both experimentally and computationally, binary complexes of Na+ and Cl− with all-cis C6H6F6 and indicated their strong binding of about 1.5−2 eV (and even a comparable binding of a second molecule on the other side).13 This supports the anticipated stabilization of the corresponding ternary systems studied in the present work. Further support comes from the previous study of similar complexes of smaller all-cis cyclic molecules, Li− C3H3F3−F and M−C4H4F4−X (M = Li−K, X = F−Br),14 exhibiting significant stability to fragmentation as well.

Figure 1. Optimized geometry and calculated electron density of C6H6F6 (cutoff of 0.05 au).

2. COMPUTATIONAL PROCEDURE Calculations have been carried out at the MP2 level of theory with the extensive basis sets aug-cc-pVTZ for H, C, and F atoms and Stuttgart’s RLC relativistic effective core potentials and associated basis sets for Cl, I, and Cs, listed in the EMSL Basis Set Exchange (https://bse.pnl.gov/bse/portal).15,16 This approach is tested to provide a very good comparison of predicted and experimental ionization energies and electron affinities of the atoms as well as of dissociation energies, equilibrium distances, and dipole moments of the CsX diatoms. In particular, IE(Cs) = 3.75 (3.89) eV, EA(I) = 3.14 (3.06) eV, and EA(Cl) = 3.44 (3.61) eV, with the bracketed values available from experiments,17,18 and the diatomic parameters are listed in Table 1. In addition, the dipole moment of the allcis C6H6F6 molecule is calculated to be 5.94 D compared to the reported 6.2 D.4 The deviations do not exceed 4% and are frequently less.

Figure 2. Optimized geometries of (top) Cs−C6H6F6−I and Cs− C6H12−I and (bottom) C6H6F6−CsI and CsI−C6H6F6.

Table 1. Equilibrium Parametersa of Ion Pair Diatoms

a

MX

De /eV

Re /Å

μe /D

CsI CsCl

3.59 (3.51 ± 0.02) 4.45 (4.62 ± 0.08)

3.38 (3.32) 2.93 (2.91)

12.3 (11.7) 10.7 (10.4)

Table 2. Equilibrium Parameters (in eV and Å) of the Studied Complexes De

system Cs−C6H6F6−I CsI−C6H6F6 Cs−C6H12−I Cs−C6H6F6−Cl CsCl−C6H6F6 Cs−C6H12−Cl Cs+−C6H6F6 C6H6F6−I− Cs+−C6H12 C6H12−I− C6H6F6−Cl− C6H12−Cl−

Bracketed values are experimental data.16,17

The NWChem quantum chemistry package19 has been employed for the specified ab initio computations. All-atom unconstrained optimizations from various initial geometries have been followed by a vibrational frequency analysis to verify the local minima of energy or transition states. When needed, the transition-state geometries have then been modified in accord with the normal-mode vectors and reoptimized. The energy barriers have been evaluated from a series of fixed displacements along the reaction coordinate with the rest of the coordinates optimized. The atomic charges have been obtained within the natural bond orbital formalism.20

a

4.50, 4.34, 2.68, 5.24, 5.31, 3.26, 1.24 1.47 0.43 0.42 1.80 0.53

0.91a 0.75a −0.91a 0.79a 0.86a −1.19a

Re (M−F/H) 2.83 2.64, 2.76 2.83 2.64, 2.73 2.96

Re (H−X)

Re (M− X)

2.63 2.83 2.49, 2.99 2.28 2.50 2.13, 2.57

7.47 3.43 6.65 7.04 2.98 6.29

2.74 2.84, 2.89 3.01, 3.03 2.37 2.60, 2.62

M−mol−X → M + mol + X, → mol + MX.

shape remains almost intact, with the most noticeable variations occurring for the tripod C−F bonds stretching by 0.04 Å and converging their F ends (adjacent to Cs) by 0.05 Å. The system is bound by about 0.9 eV relative to CsI + C6H6F6, apparently due to an overcompensation of the weakened Cs−I attraction (due to stretch) by the two ion−dipole interactions. The latter interactions can be estimated by adding to the mentioned De the difference between De(CsI) from Table 1 and D(Cs−I) = 1.35 eV calculated for the distance in the complex, resulting in 3.15 eV, which thus dominates the total binding in the system. For the Cs−C6H12−I counterpart, however, such a geometry corresponds to a very shallow local minimum of energy, separated by a negligible barrier from each of three equivalent lower-energy (by about 0.1 eV) configurations with Cs at the hollow between the inner and outer H atoms (Figure 2). The I

3. RESULTS AND DISCUSSION 3.1. Structures and Stabilities. The calculated shape and electron density distribution of C6H6F6 (hereafter its all-cis isomer) are shown in Figure 1. The C6 carbon ring creates a curvy “bagel” able to accommodate counterions along its axis, and the protruding F’s and H’s make “tripods” with hollows able to hold the ions. The same applies to the C6H12 counterpart. For the two molecules, the distance between the tripod H atoms is about 2.5 ± 0.1 Å, close to that in cyclic C3H6 studied previously.6 The predicted Cs−C6H6F6−I complex, with Cs on the F’s side and I on the H’s, is shown in Figure 2. The counterions in effect trap the molecule axially in between, with the Cs−F and H−I distances close to one another (Table 2). The molecular B

DOI: 10.1021/acs.jpca.7b02576 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

between the two interacting dipoles. The attached diatom slightly stretches (by about 0.05 Å) relative to the isolated species. For all conformers, the calculated harmonic zero-point corrections to the dissociation energies are small, within 0.02 eV, because the framing atoms are heavy, and for both the complex and its fragments, the zero-point energies are dominated by that for the molecule. With Cl replacing I, both Cs−C6H6F6−Cl and CsCl−C6H6F6 are more stable to dissociation (Table 2), apparently due to the smaller size of Cl and thus shorter Cs−Cl and Cl−molecule distances and, hence, a stronger attraction. In particular, this also applies to Cs−C6H12−Cl. The CsCl-attached isomer is, however, slightly (by about 0.1 eV) more strongly bound than the trapped one, opposite to the situation for the I-based counterparts. This is apparently due to a larger strain in (shorter) CsCl upon insertion of the molecule. While CsCl and CsI stretch about equally in the respective complexes, the loss of stability is smaller for CsI, which has a shallower well (Table 1). The same reason explains why Cs−C6H12−Cl is higher in energy relative to CsCl + C6H12 as compared to the I-based counterparts. Similar to the I-based complexes, Cs−C6H12−Cl is less stable to dissociation than Cs−C4H8−Cl.7 The former complex also shows a lower stability as compared to Li− C6H12−Cl,8 consistent with larger Cs and therefore longer Cs− Cl separation. The barriers for Cs to recombine with Cl by going around C6H6F6 and C6H12 are about 0.8 and 0.2 eV, respectively, nearly the same as that for the I-based counterpart. Consistent with the relative stabilities of Cs−C6H6F6−I and Cs−C6H6F6−Cl, both of these complexes are less stable to fragmentation than the M−C4H4F4−X (M = Li, Na; X = F− Br), K−C4H4F4−F, and Li−C3H3F3−F counterparts,14 apparently due to the bulkier trapped molecule as well as larger framing atoms and, hence, the longer separations between them. Exceptions seem to occur for K−C4H4F4−Cl and K− C4H4F4−Br bound more weakly than Cs−C6H6F6−Cl, which could likely be related to the smaller dipole of C4H4F4. In turn, the present larger systems are more stable to dissociation into molecule + MX, apparently due to a larger loss of binding in smaller MX by insertion of the molecule (similar to CsCl relative to CsI), up to a metastability of Li−C3H3F3−F. 3.2. Charges and Dipoles. The calculated charge distribution indicates a full electron transferred from Cs and concentrated mostly on I for both systems. Only about −0.20 and −0.15 e reside on C6H6F6 and C6H12, respectively. Hence, the polarity of the trapped molecule has a small effect here. In particular, the charge distribution in Cs−C6H12−I is similar to those in Cs−C4H8−I and Cs−C3H6−I, with about the same charges on the trapped molecules.6,7 Also, in Cs−C6H6F6−I, the molecule is charged similar to the cases of Li−C3H3F3−F and Na−C4H4F4−F.14 Interestingly, the polarization of the molecule by the counterions is clearly seen in the positive charges on the H atoms of C6H12, gradually increasing from the Cs to the I end and about doubling in value from the nearest to the farthest from Cs+ (Table 3). The molecule is less charged with CsI attached, by less than half as compared to the trapped isomer. Such a strong separation of charges between the framing ions spread apart by the trapped molecule leads to very large dipole moments, reaching about 25 D for Cs−C6H12−I, that is, about twice that for isolated CsI. The value is slightly larger (by 0.5 ± 0.2 D) than that for Cs−C4H8−I and Cs−C3H6−I,6,7 in accord with the Cs−I distances. The corresponding value for Cs− C6H6F6−I is somewhat smaller (Table 3) due to the oppositely

atom is also slightly shifted off-axis, following Cs, by 0.5 Å in the H−I distances (Table 1). The molecular shape distortions are more pronounced than those in the previous case, with the H atoms adjacent to Cs diverging by about 0.05 Å and those adjacent to I converging by up to 0.2 Å (unlike in Cs−C6H6F6− I, where no such convergence is found). The C−H bonds remain almost unchanged, stretching within 0.01 Å in both complexes. The reason for the more stable nonaligned geometry is the similar feature in the Cs+−C6H12 interaction, as described later, rather than the slightly shorter (by about 0.3 Å) Cs−I distance allowing one to gain the counterion attraction. No conformers with similar off-axis displacements of I are found. For the nonpolar C6H12, the complex is about 0.9 eV metastable, that is, higher in energy than CsI + C6H12, due to a weaker attraction in stretched CsI. This is similar to Cs−C4H8−I, for which the excess energy is lower by a half due to the thinner molecule in between,7 and different from Cs− C3H6−I, which can be even weakly stable (by about 0.2 eV) relative to such a dissociation,6 apparently due to a smaller distortion of the trapped molecule as well. In particular, for the Cs−I distance in the Cs−C6H12−I complex, we obtain D(Cs− I) = 1.61 eV, leaving 1.07 eV for two ion-induced dipole interactions. The complex is also slightly less stable (by 0.2−0.3 eV) to dissociation into Cs + C6H12 + I than the C3H6 and C4H8 based counterparts, consistent with a longer Cs−I distance (by 0.3−0.4 Å). In turn, a similar nonaligned geometry (or rather three equivalent ones), with Cs at the hollow between two inner and one outer F atoms, corresponds to a local energy minimum of Cs−C6H6F6−I. It is about 0.1 eV above the aligned conformer and separated from it by a barrier of only about 0.1 eV. From the aligned geometry, the energy barriers for Cs+ and − I to go around C 6 H 6 F 6 toward the counterion for recombination are evaluated as about 0.7 and 1 eV, respectively. Both values are significantly larger than the respective values of about 0.2 and 0.3 eV for the C6H12 case, which reflects the stabilizing effect of the ion−dipole interactions. In turn, the latter values are comparable to those for the C4H8 case (with larger distances between the barrier-forming H atoms) and exceed those for the C3H6 case (with about the same such distances).7,6 This is apparently due to the larger area of C6H12 and thus the longer separation between the counterions when one of them passes the edge of the molecule, which corresponds to the barrier top. The overall potential energy profile is shown in Figure 3.

Figure 3. Calculated potential energy profile of the CsI + C6H6F6 system.

Another conformer with the entire CsI attached axially to C6H6F6 via the I end (Figure 2) is slightly (by 0.16 eV) less stable, with the diatom bound to the molecules by 0.75 eV. The counterpart with CsI attached axially via its Cs end is still less stable by another 0.5 eV. This can be related to longer Cs−F distances (by about 0.5 Å due to larger atomic sizes) compared to H−I for the previous case and, hence, a larger separation C

DOI: 10.1021/acs.jpca.7b02576 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Table 3. Natural Atomic Charges and Dipole Moments (in e and D) of the Studied Complexes

a

system

q(M)

q(H)

q(F)

q(X)

μ

Cs−C6H6F6−I CsI−C6H6F6 Cs−C6H12−I Cs−C6H6F6−Cl CsCl−C6H6F6 Cs−C6H12−Cl

1.00 0.99 1.01 1.00 1.00 1.01

0.23, 0.26 0.20, 0.22 0.14−0.18,a 0.16−0.23, 0.26−0.29b 0.23, 0.28 0.20, 0.23 0.13−0.18,a 0.16−0.23, 0.27−0.31b

−0.38, −0.44 −0.38

−0.80 −0.91 −0.86 −0.84 −0.94 −0.88

20.0 21.4 24.6 18.3 19.8 22.8

−0.39, −0.44 −0.38

On the M side. bOn the X side.

from I− to I. The system is bound only weakly, by about 0.3 eV, and its dipole moment diminishes to about 0.5 D. Both systems recover their ground-state geometries if their spin state is returned back to singlet. 3.4. Pair Interactions and Nonadditivity. In order to analyze the pair interaction contributions, the two-body complexes of each molecule with Cs+ and I− were also investigated. Their optimized geometries preserve the axial position of each ion relative to C6H6F6 or recover it for the anion in the C6H12 case, while the cation remains at the 3H hollow (Figure 4). The latter conformer is, however, absent for

directed dipole of the trapped molecule. In the other conformer, this dipole is co-directed with that of CsI attached to the molecule axially, resulting, however, in an intermediate total value because the Cs−I distance is much smaller. The dipoles of the smaller M−C4H4F4−X and M−C3H3F3−X counterparts are smaller as well,14 consistent with the shorter M−X distances. The negative charges on Cl in the Cl-based complexes are slightly larger than thoes on I (Table 3), consistent with the higher electronegativity of chlorine. Accordingly, the trapped molecule is slightly more neutral. Nevertheless, the dipole moments are slightly smaller, apparently due to the shorter Cs−Cl distances. Again, the longer Cs−Cl separation in Cs− C6H12−Cl compared to that in Cs−C4H8−Cl results in a slightly larger dipole moment. The total dipole moments of the complexes are apparently dominated by the contributions from the stretched Cs−X pairs. In particular, calculations for Cs−X frozen at the distance in Cs−C6H6F6−I, Cs−C6H6F6−Cl, Cs−C6H12−I, and Cs− C6H12−Cl give dipoles of 34.8, 33.0, 30.5, and 29.2 D, respectively. These values exceed those of the respective complexes by about 14.7 and 6.2 ± 0.2 D for the C6H6F6- and C6H12-based systems, respectively. The larger reduction caused by the polar insert is clearly due to its counter-directed dipole and almost halves the M−X value, while the nonpolar insert decreases it by about a quarter. These considerable changes indicate the significance of the induced dipoles on both the molecule (prevailing for the C6H12 case) and the framing atoms (for the C6H6F6 case). Of course, the fractional charge caught by the molecule also reduces the total dipole. 3.3. Triplet State. In the triplet state, Cs−C6H6F6−I stretches axially, by about 0.9 Å in Re(Cs−I), mainly via increased H−I distances (by about 0.6 Å), while the Cs−F one is only about 0.1 Å longer. This is related to neutralization of the framing atoms in this state, as for isolated CsI. Unlike for CsI, however, the atoms are still held by the large dipole of the molecule via polarization, which explains the above larger withdrawal of the less polarizable I atom. Consistently, the total dissociation energy diminishes to about 0.7 eV, and the system becomes metastable relative to CsI + C6H6F6. As another consequence, the dipole moment turns around and reduces by a third (to about 13 D) and remains twice that for the isolated molecule, apparently contributed to significantly by induced dipoles on the framing atoms. In comparison, the CsI−C6H6F6 isomer in the triplet state would keep the same orientation of the reduced dipole. In contrast, Cs−C6H12−I in the triplet state generally preserves its nonaligned geometry with off-axis Cs. Opposite to the case above, it is this atom that withdraws significantly (by about 1.5 Å, more strongly than that from polar C6H6F6), while I remains at about the same distance. This appears consistent with the larger relative size change from Cs+ to Cs than that

Figure 4. Optimized geometries of (top) Cs+−C6H6F6 and C6H6F6− I− and (bottom) Cs+−C6H12 and C6H12−I−.

Cs+−C6H6F6. The conformer of Cs+−C6H12 with the axial cation remains, is only 0.06 eV higher in energy and separated by a tiny barrier. In the absence of the counterion on the other side, the ions are, expectedly, somewhat farther away from the molecule, by only about 0.1 Å (in terms of the Cs−F and H−I distances) for the C6H6F6 case and by 0.2−0.5 Å for the C6H12 case (Table 2). This is consistent with the stronger ion−dipole interactions and, hence, a weaker effect of the ion−ion remote attraction in the former case. The binding energies of the ions are relatively low for C6H12, under 0.5 eV and almost degenerate for the cation and anion. For C6H6F6, they are significantly higher, up to 1.5 eV, and are larger for the anion case (Table 2). This is consistent with the respective ion-induced dipole and ion−dipole-dominated interactions in the systems (clearly reflected also in the different H−I distances for the C6H6F6 and C6H12 cases), as well as with the close sizes of Cs+ and I− plus the longer C−F bonds as compared to C−H (hence, Cs+ is slightly farther away from C6H6F6). A previous study of Na+−C6H6F613 predicted about 0.5 eV stronger binding at about a 0.7 Å shorter Na−F distance as D

DOI: 10.1021/acs.jpca.7b02576 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A compared to the present Cs+−C6H6F6 cation, consistent with the smaller size of Na. In the earlier work involving M+−C6H12 (M = Li, Na, K),8 the binding energies decreased (and M−H distances increased) with increasing size of M, reaching values already lower than those for M = Cs in the present work. There, however, the metal cation was located centrally, which could correspond to slightly weaker binding, according to the present work. In comparison, M+−C4H4F4 (M = Li, Na)14 are predicted to be more strongly bound than Cs+−C6H6F6, again consistent with smaller M, while the more weakly bound K+− C4H4F4 could perhaps be a manifestation of the smaller dipole of the molecule. The C6H6F6−Cl− and C6H12−Cl− anions are more compact and stable than the corresponding I-based counterparts, consistent with smaller Cl. The H−Cl distance is shorter by about 0.4 Å for both, and the binding energies are larger by about 0.3 and 0.1 eV, respectively (Table 2). In particular, the results for C6H6F6−Cl− are in accord with the previous work using a DFT-optimized geometry13 and predicting about 0.3 eV weaker binding at about 0.1 Å longer H−Cl separation. For C6H12−Cl−, an earlier study predicted slightly weaker binding as well.8 In comparison, the binding energy of C6H6F6−I− is in between those for C4H4F4−X− (X = F−Br),14 while the value for C6H6F6−Cl− is above those, again reflecting the interplay between the relative dipole moments of the molecules and their relative separations from the ions. By adding together the binding energies for the three pair interactions calculated for geometries as in Cs−C6H6F6−I (including stretched Cs−I, relative to Cs+ + I−), we obtain a combined value 0.63 eV smaller than the total De value of the entire system. Therefore, we could identify cooperative nonadditive interactions here, which could be interpreted, for example, in terms of the attractions of the ion-induced dipoles with the counterions. From such “internal” geometries, the isolated Cs+−C6H6F6 and C6H6F6−I− species relax only slightly, by about 0.1 eV in energy. In comparison, a similar analysis for Cs−C6H12−I leads to a slightly lower cooperativity of the interactions, at 0.40 eV. The higher cooperativity in Cs− C6H6F6−I could be related to the interactions of ion-induced dipoles of the counterions with the polar molecule and the other way round, dipole-induced dipoles on the ions interacting with the counterions. For Cs−C6H6F6−Cl, the cooperativity of 0.65 eV in binding energy is nearly the same as that for the I-based counterpart. These values are intermediate between those obtained for Li− C3H3F3−F and Na−C4H4F4−F. The value of 0.49 eV for Cs− C6H12−Cl is slightly above that for the analogous system with I and is about twice as large as that for the Li-based counterpart.8 The latter is likely due to a higher charge on Cs, consistent with its lower ionization energy, as well as due to its higher polarizability. In both present C6H12-based species, the attraction between the framing ions dominates the system stability. In the C6H6F6-based counterparts, however, the sum of the two ion−molecule interactions prevails. The cooperativity of pair interactions can also be analyzed relative to the actual dissociation limit with neutral atomic components, M + molecule + X. The sum of binding energies of Cs−C6H6F6, C6H6F6−I, and Cs−I for their geometries inside of Cs−C6H6F6−I is found to be lower than the De value of the complex by 3.2 eV (which value even exceeds this sum) and about 5 times larger than the value obtained for the ionic asymptote above. In contrast, for the C6H12-based counterpart, the extra stabilization is only about 1.0 eV, that is, 2.5 times that

for the ionic asymptote. This further exhibits the significance of the interactions between the framing ions and the trapped polar as compared to nonpolar molecule for the system stabilization. For the Cs−C6H6F6−Cl case, the De value exceeds the sum of neutral pair interactions even more, by 3.7 eV, consistent with the larger stability of the complex as compared to the Ibased counterpart. This is an almost 6-fold increase from the value for the ionic asymptote. However, for Cs−C6H12−Cl, the cooperativity of binding amounts to 1.3 eV, that is, almost 3 times that for the ionic asymptote and more than triple the value for Cs−C6H12−I. 3.5. IR Intensity Spectra. Calculated spectra of infrared intensities (Figure 5) show some similarities as well as significant differences for isolated C6H6F6 and its CsI-trapped and -attached complexes.

Figure 5. Calculated IR intensities of (top to bottom) C6H6F6, CsI− C6H6F6, and Cs−C6H6F6−I.

The strongest IR line (predicted near 3000 cm−1) for Cs− C6H6F6−I corresponds to stretching of the three inner CH groups parallel to the system axis (toward the I atom). This mode is more than an order of magnitude more intense and red-shifted by about 100 cm−1 relative to the corresponding vibrations in isolated C6H6F6. The second strongest mode (near 1200 cm−1) represents the symmetric vibrations with these CH groups moving perpendicular to the system axis (to− from it) while keeping near-parallel orientation. In particular, the latter vibration corresponds to the most IR-intense one in the isolated molecule, slightly red-shifted (by about 50 cm−1) and almost tripled in intensity in the complex. Two neardegenerate second most intense lines in C6H6F6, corresponding to the outer CH groups bending perpendicular to the axis (rocking and scissoring modes), exhibit a similar evolution in the complex, where they are represented by the third strongest lines (near 1100 cm−1), though the red shift is smaller and the intensity increases weakly. In all of above cases for Cs− C6H6F6−I, the Cs and I atoms are relatively motionless. In comparison, in CsI−C6H6F6, the above three most intense lines of C6H6F6 red shift by only about 20 cm−1, increasing in intensity slightly for the two degenerate modes and nearly twice E

DOI: 10.1021/acs.jpca.7b02576 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

additive stabilization relative to the sum of pair contributions, by about 0.5 eV for the ionic asymptote (M+ + molecule + X−) and a few eV for the neutral one (M + molecule + X). Unlike in the aligned Cs−C6H6F6−X systems, the Cs atom is positioned off-axis in the Cs−C6H12−X species. An interesting feature associated with the latter case is the possible delocalization of Cs around the aligned geometry (which in effect corresponds to a low barrier of about 0.1 eV). The counterions, considerably separated by the trapped molecules, dominate the large dipole moments of the complexes (about 20 D). In particular, this may suggest possible applications in light sensors. In addition, this illustrates a way of adding polarity to nonpolar molecules by means of attached ion pairs. Interestingly, for the inserted polar molecule, its own dipole mainly holds these ions while being counterdirected to the overall dipole of the system, hence reducing it. As a further twist, in the triplet state, the charge transfer between the framing ions is gone; therefore, the overall dipole is dominated by that of the trapped molecule and is thus directed oppositely. Such a spin-controlled dipole switch could probably find applications in molecular electronics and/or photonics. Such systems could perhaps be formed via a mechanism suggested for analogous complexes H−Rg−X in rare gas solids,21,22 produced also in clusters,23 that is, via photodissociation of MX in the clusters of the molecules involved. Moreover, polar molecules can be expected to be trapped more efficiently, in particular, via a two-body stepwise sequence, for example, M + mol → M−mol, followed by M−mol + X → M− mol−X, especially if the MX−mol isomer is less stable. This should be facilitated by significant dipole−induced dipole interactions resulting, for example, in about 0.3 eV binding for Cs−C6H6F6, according to our estimates. The analogous ionic channel via sequential attachment of the counterions, as suggested previously,14 is another possibility. When formed, the M−mol−X complexes could be identified using the IR intensity spectra. These are predicted to be sensitive indicators of the trapped and attached isomers in terms of the ratios of the IR intensities corresponding to specific vibrations of the molecule. Besides, the intensities can increase in the complex by up to an order of magnitude. This, in particular, illustrates a possible way of efficient experimental detection and characterization of nonpolar molecular species via attached ion pairs.

for the most intense one. This isomer also exhibits a weaker line near 3000 cm−1, about three times as intense as that for the isolated molecule. For C6H12, the most intense lines correspond to the stretching vibrations of both the inner and outer CH groups. In the Cs−C6H12−I complex, the strongest line near 2900 cm−1 (Figure 6) represents mostly stretching of the three inner CH

Figure 6. Calculated IR intensities of C6H12 (top) and Cs−C6H12−I (bottom).

groups positioned between the (off-axis) Cs and I atoms. It is red-shifted by about 200 cm−1 and has an order of magnitude higher intensity compared to that for the isolated molecule. The second most intense line near 3000 cm−1 in the complex is red-shifted by about 130 cm−1 and is about twice as strong as that in the molecule. The corresponding vibrations are dominated by stretching of the other three inner CH groups, which are farther away from in between Cs and I. The third strongest line near 3050 cm−1 involves stretching of all of the inner (as well as outer) CH groups and is relatively weakly shifted and only slightly more intense than that in the molecule.

4. CONCLUSIONS A group of novel insertion complexes of molecules trapped in atomic ion pairs, M−mol−X (M = Cs and X= I, Cl), their noninsertion (attached) MX−mol isomers, as well as binary structural components have been studied ab initio. The investigated trapped molecules, polar all-cis C6H6F6 and nonpolar C6H12, involve a range of noncovalent interactions in the complexes, such as ion−ion, ion−dipole, ion−induceddipole, and dipole−dipole. Relative to the MX + mol asymptote, the insertion complexes vary from stable for the polar trapped molecule to metastable for the nonpolar one, by about 1 eV in either case. The potential barrier stabilizing the metastable C6H12-based species is about 0.2 eV, which implies low-temperature conditions needed for their preservation. Furthermore, the “dipole inside of dipole” or trapped Cs−C6H6F6−I isomer can be even slightly more stable (by about 0.2 eV) than the dipole−dipole or attached CsI−C6H6F6 one, while the situation is opposite for the Cl-based analogues (with only about a 0.1 eV gap). Such systems can thus be realistic and experimentally observable. In the C6H6F6-based systems, the two interactions between the framing ions and the molecule slightly prevail in the overall binding, while in the C6H12-based counterparts, the ion−ion interactions dominate. The systems exhibit cooperative non-

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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.7b02576. Calculated total energies (in Hartree) and Cartesian coordinates (in Å) of Cs−C6H6F6−I, Cs−C6H6F6−Cl, Cs−C6H12−I, and Cs−C6H12−Cl (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +1-905-7213304. ORCID

Fedor Y. Naumkin: 0000-0001-7356-9618 Notes

The author declares no competing financial interest. F

DOI: 10.1021/acs.jpca.7b02576 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

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ACKNOWLEDGMENTS Financial support of the UOIT Faculty of Science is acknowledged. All calculations have been carried out at the high-performance computing facilities of the UOIT Faculty of Science and of the SHARCnet academic network of Ontario. The author is grateful to their staff for technical assistance.

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DOI: 10.1021/acs.jpca.7b02576 J. Phys. Chem. A XXXX, XXX, XXX−XXX