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Letter
Probing the Lone Pair···#-Hole Interaction in Perfluorinated Heteroaromatic Rings: The Rotational Spectrum of Pentafluoropyridine·Water Camilla Calabrese, Qian Gou, Assimo Maris, Walther Caminati, and Sonia Melandri J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00473 • Publication Date (Web): 07 Apr 2016 Downloaded from http://pubs.acs.org on April 9, 2016
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The Journal of Physical Chemistry Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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importance as a solvent, the water molecule was selected as a partner, as in many other studies,15,16 to probe the electron density of the counterpart and the different kind of interactions. The high accuracy on the structural data that can be achieved by a rotational spectroscopic experiment will unambiguously establish both the effects of perfluorination on the electron density at the interaction sites and the binding abilities of 5FPY. Preliminary ab initio calculations on 5FPY·W have been performed with the Gaussian09 program package,17 using second-order Møller−Plesset (MP2) theory with 6-311+ +G(d,p) basis set, and correction for basis set superposition error was included by means of the counterpoise (CP) procedure.18 Computational results suggest that the water molecule can approach 5FPY in two different ways, giving rise to two stable arrangements (see Figure 1): The water moiety
1 for the principal axes systems (PASs).) The different orientations of the dipole moment with respect to the PAS of the two isomers have significant effects on the appearance of the rotational spectra, which can then be used to unambiguously discern between the two forms. Preliminary trial searches were aimed at detecting μc-type Rbranch rotational transitions of 5FPY·W-π. The 52,3 ← 41,3 transition, shown in Figure 2, was observed first; then, the
105 106 107 108 109 110 111 112 f2
Figure 2. 523 ← 413 transition of the observed 5FPY·W-π isomer.
assignment was extended to other rotational transitions with Jupper ranging from 4 to 8. Because of the overall low intensity of the spectrum, it was not possible to measure any μb-type Rbranch transition in agreement with the smaller theoretical value (μb/μc 1:4). All measured transitions displayed a typical hyperfine structure due to the coupling of the quadrupolar 14N nucleus and the overall rotation. Moreover, all rotational transitions consisted of doublets related to two tunneling states (indicated by 0+ and 0−) generated by an internal large amplitude motion taking place in the molecular complex. The observed splittings varied from 0.16 to 5.10 MHz, and all doublets showed a stronger high-frequency component and a weaker low-frequency component (see Figure 2). The relative intensity of these two components is 3:1, which corresponds to the statistical weight expected for the internal rotation of water around its C2 axis, inverting the positions of a pair of Fermions with spin I = 1/2. For this reason, following statistical weights arguments, the weaker component belongs to the ground state (0+). A similar internal motion due to the water moiety was observed and analyzed also in the rotational spectra of phenol,19,20 benzonitrile,21 and acrylonitrile·water complexes.22 Moreover, the nature of the motion is confirmed by the investigation of the water isotopic species (5FPY·H218O, · DOH, and ·HOD). In particular, for the monodeuterated isotopologues, the splitting was not observed because the two hydrogen atoms are not equivalent anymore, while, as expected, the splitting still appears in the rotational spectrum of the 5FPY·H218O species. The measured transition frequencies were fitted with Pickett’s SPFIT program23 using Watson’s semirigid rotor Hamiltonians in the “S” reduction and Ir representation,24 which are given here in a simple form
Figure 1. Two isomers of 5FPY·W (left side: 5FPY·W-π; right side: 5FPY·W-σ) with the atom numbering used in the text and the principal inertial axes. 91 92 93 94 95 96 97 98 t1
99 100 101 102 103 104
can either lie above the 5FPY ring with the oxygen lone pairs oriented toward its center (5FPY·W-π) or in the ring plane with one hydrogen of the water pointing toward the nitrogen lone pair of the 5FPY unit (5FPY·W-σ). In 5FPY·W-π the water molecule lies in a plane perpendicular to the ring containing the nitrogen atom in a Cs symmetry, while in 5FPY·W-σ the water is nearly coplanar to the ring. The corresponding relative energies and spectroscopic parameters are shown in Table 1. It is worth noting that the latter form turns out to be a transition state if CP correction is not included. Concerning the orientation of the calculated electric dipole moment, μc represents the highest component for the 5FPY·Wπ conformation, while μa dominates in 5FPY·W-σ. (See Figure Table 1. Theoretical Spectroscopic Parameters and Relative Energies of the Two Possible Isomers of 5FPY·W (MP2/6311++G(d,p)-CP) A, B, C/MHz μa, μb, μc/D DJ, DJK, DK/kHz χaa, χbb, χcc/MHz ΔE0/kJ mol−1 a
5FPY·W-π
5FPY·W-σ
1003, 819, 608 0, −0.35, 2.26 0.22, 0.43, −0.40 2.37, −4.51, 2.15 0a
1064, 698, 423 −3.05, 1.54, −0.66 0.53, −1.30, 3.04 −4.60, 2.36, 2.24 4.94
+
−
H = HR (0 ) + HR (0 ) + HQ (N)
Absolute zero-point-corrected energy: −819.186593 hartree. B
(1)
DOI: 10.1021/acs.jpclett.6b00473 J. Phys. Chem. Lett. XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry Letters H = HR + HQ (N) + HQ (D)
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value for the a coordinates for all three atoms, which is consistent with the water molecule lying in the bc plane (a = 0), in agreement with the theoretical findings on 5FPY·W-π. Starting from the ab initio geometry, a partial r0 structure was then obtained by adjusting the O12−N1 distance (R), the O12−N1−C6 angle (α), and the C3−N1−C2 angle to reproduce the experimental rotational constants of all four isotopologues with an error of ∼1 MHz. The fitting procedure was performed using the STRFIT program.26 The parameters derived with both procedures (rs and r0) are reported in Table 3 with the theoretical (re) ones of both isomers. Comparison of the data further confirms the assignment of the observed spectrum to the 5FPY·W-π species. At the same time, it can be noted that the agreement of the hydrogen atoms’ coordinates is poor, probably due to the internal motion that affects the molecule of water with respect to 5FPY. The shape of the potential energy function of the internal motion (reported in Figure 3) was obtained at the MP2/6-
(2)
In eq 1, which refers to the parent and 5FPY·H218O species, the HR(0+) and HR(0−) represent the Hamiltonians for the 0+ and 0− tunnelling states including centrifugal distortion contributions, while HQ is the operator associated with the interaction of the 14N nuclear electric quadrupole moment with the electric field gradient at the N nucleus. For the monodeuterated isotopologues, where no vibrational effects were observed but the quadrupole hyperfine structure due to the deuterium atom was resolved, eq 2 was chosen. From the comparison of the experimental spectroscopic parameters (see Table 2) and the theoretical ones, it is possible to unequivocally Table 2. Experimental Spectroscopic Constants of 5FPY·Wπ for 0+ and 0− Tunneling States +
A/MHz B/MHz C/MHz DJ/kHz DJK/kHz DK/kHz d2/kHz χaa, χbb, χcc/MHz Nb σ/kHzc
−
0
0
1009.0860(1)a 819.3550(2) 614.1098(7) 0.292(2) 0.52(1) −0.566(8) 0.0839(3) 1.964(6), −3.720(5), 1.756(5) 116 3
1008.8572(1) 819.6669(2) 613.6164(7) 0.285(2) 0.52(1) −0.548(8) 0.0828(3)
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a
Error in parentheses in units of the last digit. bNumber of lines in the fit. cRoot-mean-square deviation of the fit.
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assign the observed spectrum to the 5FPY·W-π species. Not only the rotational constants but also the nuclear quadrupole coupling constants and the observation of a μc-type spectrum all contribute to the evidence of this assignment. Additional searches for 5FPY·W-σ were pursued, but no lines were found. The changes in the principal moments of inertia resulting from isotopic substitution allow the determination of the molecular structure of the investigated system. From the full set of the experimental rotational constants (12 values), a partial experimental rs structure was determined using Kraitchman’s equations;25 the absolute values of the rs coordinates for the three water atoms are given in Table 3. (See Figure 1 for the numbering of atoms.) The rs procedure led to a small imaginary
Figure 3. r0 structural parameters of 5FPY·W-π. The black point lies on the symmetry axis of the ring at the mid distance between the N and the opposite C atoms. The graph represents the potential energy function described by τ. Points and dotted line represent the ab initio data and the fitted function, respectively; continuous line is the adapted function from the flexible model.
311++G(d,p) level of theory. The procedure explored the coordinate τ indicated in the same Figure, with a step size of 15°. The geometry of both water and 5FPY moieties was fixed
Table 3. Comparison between Experimental (rs and r0) and Calculated (re) Structural Parameters of the 5FPY·W Isomers rsa a(O12)/Å b(O12)/Å c(O12)/Å a(H13)/Å b(H13)/Å c(H13)/Å a(H14)/Å b(H14)/Å c(H14)/Å RO12−N1/Å αO12−N1−C6/deg
