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Effects of Fluorine Substitution on the Microsolvation of Aromatic Amines: The Microwave Spectrum of 3-Fluoropyridine-Water Camilla Kumar Calabrese, Qian Gou, Lorenzo Spada, Assimo Maris, Walther Caminati, and Sonia Melandri J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b00785 • Publication Date (Web): 09 Mar 2016 Downloaded from http://pubs.acs.org on March 16, 2016

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The Journal of Physical Chemistry

Effects of Fluorine Substitution on the Microsolvation of Aromatic Amines: The Microwave Spectrum of 3Fluoropyridine-Water Camilla Calabrese,‡ Qian Gou,‡,# Lorenzo Spada,‡ Assimo Maris,‡ Walther Caminati,‡ Sonia Melandri‡,* ‡

Dipartimento di Chimica “G. Ciamician” dell’Università, Via Selmi 2, I-40126 Bologna, Italy.

#

Present address: School of Chemistry and Chemical Engineering, Chongqing University,

Chongqing, P. R. China, 400030.

Corresponding Author: *Sonia Melandri, Email: [email protected]

ACS Paragon Plus Environment


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ABSTRACT: The rotational spectra of the parent species and of four water isotopologues (-DOH, -HOD, -DOD and -H18OH) of the adduct 3-fluoropyridine-water have been investigated using pulsed supersonicjet Fourier-transform microwave spectroscopy. From the rotational constants, the structure of the adduct was deduced where the water is linked to the aromatic ring trough an intermolecular OH···N hydrogen bond with a bond distance of 1.9961(5) Å and O-H···N angle of 156.8(1)°. The shape of the complex is such that the water oxygen is in the plane of the aromatic ring, on the opposite side of fluorine, and forming a C-H···N weak hydrogen bond with the adjacent aromatic hydrogen.


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1. Introduction Pyridine (PY) is one of the most important heterocyclic aromatic molecules and it continues to be an important topic of research which is present in all areas of chemistry and biochemistry related journals with more than 30000 article published in the last five years. PY and a large number of its derivatives are used extensively in coordination and surface chemistry, and for this reasons, as described by Gou and co-workers,1 many of its molecular adducts have been studied using high resolution molecular spectroscopy. In particular rotational spectroscopy has proved to be a fundamental tool to obtain structural information and to describe intermolecular interactions such as van der Waals interactions and the hydrogen bond. As regards to possible derivatives, the exploitation of fluorine chemistry in all fields of science opens up many new opportunities to modify molecular properties difficult to achieve with other substituents. In fact the substitution of a hydrogen atom with fluorine can have a strong influence on the properties of pharmaceutical compounds2 and novel materials3 because of its size and stereoelectronic effects, and it can profoundly change the conformational preferences of a molecule and its interactions with the surroundings in particular those involved in the solvation process. Gas-phase spectroscopic studies on small aromatic molecule-(H2O)n clusters, generated in a supersonic molecular expansion, are particularly relevant to explore the forces that drive the solvation process of aromatic rings. In order to give information on the effect of fluorine substitution on the solute molecules, we focused our attention on the molecular adducts of water with fluorinated derivatives of PY, investigated with rotational spectroscopy. In this work we consider the model system 3-fluoropyridine-water (3FPY-W), where fluorine is in the meta position with respect to the nitrogen atom. The monomer, 3-fluoropyridine (3FPY), was already characterized spectroscopically both with rotational4 and vibrational techniques.5 Regarding rotational spectroscopy in supersonic expansion, the most recent work presented by van Dijk et al.,4 reporting the analysis of the spectra of the 3FPY ACS Paragon Plus Environment


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parent species and its six heavy atom isotopologues, provides the effective structure of the molecule. In agreement with Boopalachandran et al.,5 the authors find that fluorination in the meta position has a smaller effect on the geometry of the PY than the introduction of fluorine in ortho position (2fluoropyridine, 2FPY). Seeking information about the fluorination effect on intermolecular interactions, we can compare the van der Waals complexes formed by argon with PY,6 2FPY7 and 3FPY.7 In the first case the argon atom lies above the aromatic ring, in the symmetry plane perpendicular to PY, and it is oriented towards the nitrogen atom, because of the higher electron density at this site.6 Also in the fluorinated pyridine compounds the argon atom lies above the ring and its location is determined by the balance of the attractive forces of both heteroatoms. It is interesting to note that, due to the depletion of the electron density on the ring caused by fluorination, the pseudo-diatomic stretching force constants become smaller in going from PY to 2FPY and 3FPY. Regarding the interaction of PY with solvent molecules, a recent paper by Nieto et al.,8 in which the authors characterize the formation of the pyridine-water (PY-W) heterodimer with IR spectroscopy performed in helium nanodroplets, indicates that the most stable structure is the O-H···N bonded one. This result is in agreement with those obtained by rotational spectroscopy on the 1:1 adducts of water with diazines

9-11

and 1,3,5,-triazine.12 The rotational spectrum of the complex PY-W

observed and assigned in our laboratory, is rather complicated, due to the 14N quadrupole coupling splittings and the internal motions of the water moiety connecting two equivalent configurations. For this reason its full assignment has not yet been completed. The fluorination in meta position of the PY ring will break the symmetry and therefore simplify the internal motion pathway of water. Weak hydrogen bond interactions with PY were investigated by means of rotational spectroscopy, using several fluorinated probes in the studies of the PY-CHF3,13 PY-CH3F14 and PY-CH2F215 complexes. In the case of PY-CHF3 and PY-CH3F the freon points toward the nitrogen atom lone pair, forming a C-H···N weak hydrogen bond while one fluorine is oriented towards the hydrogen


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atom adjacent to the nitrogen, engaging in another intermolecular interaction. Besides the presence of this double interaction, the rotational spectra of both complexes show evidence of the fluorineorganic molecule undergoing an internal rotation motion. Differently, in PY-CH2F2 three weak hydrogen bonds take place (a bifurcated CH···N and a CH···F one ) quenching any internal motion of the CH2F2 moiety. The -type bonds observed in the complexes of PY with freons differ from the C-H···-type interaction observed in benzene-CHF3,16 demonstrating the greater preference of such molecules to interact with the nitrogen atom lone pair instead of the aromatic -cloud. Concerning the interaction of water with fluorinated aromatic compounds, the cases of 1:1 fluorobenzene-water and p-difluorobenzene-water were studied by rotational spectroscopy.17 For both complexes, the results indicate a planar structure where water is σ-bound to the fluorobenzene species forming a six-membered hydrogen bonded ring. This behavior differs from what was observed for the water complex of the unsubstituted benzene molecule,18 where water lies above the ring plane with both hydrogen atoms pointing toward the π-cloud, undergoing a nearly free internal rotation. In the present work we report on the observation and analysis of the pure rotational spectrum of 3FPY-W. Pulsed supersonic-jet Fourier Transform Microwave (FTMW) spectroscopy was used to obtain information about the conformational preferences of this system in its ground vibronic state. The spectra of the enriched water isotopic species were also recorded in order to obtain more detailed structural information to assess the relative orientation of the two moieties. 2. Experimental section 3FPY (98%) was purchased from Alfa Aesar and used without further purification. It appears as a colourless liquid and its boiling point is 107°C. D2O and H218O were supplied by Cambridge Isotope Laboratories, Inc.



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Molecular clusters were generated in a supersonic expansion, under conditions optimized for the 1:1 molecular adduct formation. The measurements were carried out using helium as carrier gas at a stagnation pressure of ∼0.5 MPa which was passed over the samples of water (at room temperature) and 3FPY (at 0°C) kept into separate containers and expanded through a solenoid valve (General Valve, Series 9, nozzle diameter 0.5 mm) into the vacuum chamber. The rotational spectra in the 6-18.5 GHz frequency region were recorded using a COBRA-type19 pulsed supersonic-jet Fourier-transform microwave (FTMW)20 spectrometer, described elsewhere.21 The spectral line positions were determined after Fourier transformation of the time domain signal with 8k data points, recorded with 100 ns sample intervals. Each rotational transition displays a Doppler splitting, enhanced by the coaxial arrangement of the spectrometer. The rest frequency was calculated as the arithmetic mean of the frequencies of the two Doppler components. The estimated accuracy of the frequency measurements is better than 3 kHz, resolution is better than 7 kHz. 3. Theoretical methods Geometry optimization of different structures of 3FPY-W has been performed using the MP2 method with the 6-311++G(d,p) basis set. Subsequent vibrational frequency calculations were performed in the harmonic approximation to check whether of all the achieved stationary points were real minima. To correct the basis set superposition error (BSSE) effects, the same kind of calculations were also run using the counterpoise procedure.22 The Gaussian 09 program package was used to carry out all the calculations.23 4. Results and Discussion 4.1. Theoretical Results 3FPY possesses three different proton-acceptor sites: the π cloud and the lone pairs on the nitrogen and the fluorine atoms. Previous studies on microsolvation of fluorobenzenes17 and diazines9-12 show that the water is hydrogen bonded to the nitrogen or to fluorine in an in-plane arrangement


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and that a secondary interaction between the water oxygen and the closest ring hydrogen atom occurs. Applying these considerations to the 3FPY-W system, the four structures reported in Table 1 can be drawn. When the O-H···N interaction is considered, the O atom can be directed towards the F atom (3FPY-W_2) or opposite to it (3FPY-W_1) and when a O-H···F interaction takes place, the O atom can point towards the N atom (3FPY-W_4) or in opposite direction (3FPY-W_3). The four conformers were optimized by means of ab initio calculations achieving the results summarized in Table 1. The global minimum (3FPY-W_1) presents an intermolecular O-H···N hydrogen bond with the oxygen atom in the PY plane oriented opposite to the fluorine atom, pointing towards the closest C-H group. From Table 1 it can also be seen that in one case (3FPYW_2) the inclusion of BSSE correction in the calculations leads to different shapes. However, the relative energies’ order for the optimized minima is the same with both methods which leads to 3FPY-W_1 being the global minimum. It is worth noting that, from the comparison of the dissociation energies, the adduct in which a hydrogen bond is formed with the F atom results less stable (about 10 kJ mol-1) than the one where the water is linked to the N atom. Table 1. MP2/6-311++G(d,p) calculated spectroscopic parameters and relative energies of the 3FPY-W conformers (without/with BSSE corrections). 3FPY-W_1

3FPY-W_2a

3FPY-W_3

3FPY-W_4

/ A/MHz

4159/4096

3390/2900

3989/4017

3892/3936

B/MHz

1067/1046

1129/1254

1200/1168

1213/1174

C/MHz

852/835

852/877

923/905

925/904

µa/D

-2.36/-2.24

3.25/-2.60

2.03/1.98

-0.34/-0.42

µb/D

0.98/1.02

2.31/3.46

0.74/0.68

2.81/3.02

µc/D

1.15/1.14

1.59/1.25

0.84/0.00

0.88/0.03



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χaa/MHz

-2.96/-3.74

-4.07/-5.49

-3.39/-4.03

1.54/1.62

χbb/MHz

-0.11/-0.10

1.04/1.65

0.05/-0.09

-4.94/-5.81

χcc/MHz

3.08/3.84

3.04/3.84

3.34/4.13

3.4/4.20

Paa/uÅ2 b

472.7/482.6

445.7/402.4

421.0/432.6

416.5/430.5

Pbb/uÅ2

120.5/122.9

147.1/173.7

126.5/125.8

129.6/128.4

Pcc/uÅ2

1.0/0.5

2.0/0.6

0.2/0.0

0.3/0.0

∆E/kJ mol-1

0c/0d

0.4/0.3

10.9/10.4

14.5/13.8

∆E0/kJ mol-1

0e/0f

0.5/0.3

9.0/8.2

12.2/11.0

De/kJ mol-1

29.2/23.6

28.9/23.3

18.4/13.2

14.7/9.8

D0/kJ mol-1

21.8/16.4

21.3/16.1

12.7/8.2

9.6/5.4

a

Species 3FPY-W_2 changes from the left to the right shape when including BSSE corrections. bPaa, Pbb, Pcc :

planar moments of inertia; definition is given in the text. c,d,e,f Absolute energies are -422.970834, -422.96868, 422.866522, -422.86633 Eh, respectively.

4.2 Rotational spectra Preliminary trial predictions of the spectra were based on both sets of calculated rotational constants of 3FPY-W_1. According to the theoretical values of the dipole moment components, the first search has been targeted to the µa-R band. The J = 4←3 was observed first and then the assignment was extended to Jupper up to 9.


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Figure 1. The 404 ←303 transition of the observed 3FPY-W_1 conformer. Each rotational line is split by Doppler effect and the nitrogen nuclear quadrupole hyperfine structure is resolved (quantum numbers F’ ← F are shown).

After the assignment of the first µa-R lines, it was possible to record some µb-type transitions, with rotational quantum number J ranging from 3 to 7 for a total of 41 rotational transitions. In addition, also some µb-Q branch transitions were observed. All the measured transitions showed a nuclear-quadrupole hyperfine structure due to the presence of the nitrogen

14

N nucleus (see for

example 404 ←303 transition in Figure 1). All experimental frequencies (given as Supporting Information) were fitted using Pickett’s SPFIT program24 within the S-reduction of Watson’s semirigid rotational Hamiltonian in the Ir representation,25 providing the spectroscopic constants reported in the first column of Table 2: rotational constants (A, B, C), centrifugal distortion constants (DJ, DJK, DK, d1, d2), quadrupole coupling constant (aa, bb, cc) and planar moments of inertia (Paa, Pbb, Pcc). The experimental spectroscopic constants show that the MP2/6-311++G(d,p) calculation without BSSE correction is more accurate. Table 2. Experimental spectroscopic constants of the observed isotopologues of water in 3FPYW_1 (S-reduction, Ir representation). -HOH a

-H18OH

-DOH

-HOD

-DOD

A/MHz

4163.0585(9)b

4148.419(2)

4136.381(3)

4123.842(3)

4098.341(3)

B/MHz

1064.3651(1)

1003.1465(2)

1047.8077(4)

1024.6943(4)

1009.6845(4)

C/MHz

847.8457(1)

807.9682(2)

836.2324(5)

821.3607(4)

810.7445(4)

DJ/kHz

0.1309(9)

0.126(2)

0.122(5)

0.122(4)

0.116(4)

DJK/kHz

4.90(1)

[4.90]c

[4.90]

[4.90]

[4.90]

DK/Hz

21.03(9)

[21.03]

[21.03]

[21.03]

[21.03]

d1/Hz

-41.3(8)

[-41.3]

[-41.3]

[-41.3]

[-41.3]

d2/Hz

-25.4(6)

[-25.4]

[-25.4]

[-25.4]

[-25.4]

χaa/MHz

-3.085(8)

-3.12(3)

-3.04(7)

-3.15(7)

-3.08(7)

χbb/MHz

-0.119(5)

-0.086(1)

-0.097(3)

-0.106(3)

-0.099(3)

χcc/MHz

3.204(5)

3.204(1)

3.135(3)

3.255(3)

3.182(3)



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Paa/uÅ2 d

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474.74776(4)

503.7315(1)

482.2468(2)

492.9721(2)

500.2851(2)

Pbb/uÅ

121.32651(4)

121.7622(1)

122.1055(2)

122.3228(2)

123.0666(2)

Pcc/uÅ2

0.06958(4)

0.0623(1)

0.0735(2)

0.2277(2)

0.2465(2)

2

e

N

σf/kHz a

135

66

30

33

33

3

3

3

4

4

In the HOH notation, the first and second water hydrogens correspond to H12 and H14 of Figure 2, respectively.

b

Error in parentheses in units of the last digit. c Values in brackets fixed to the corresponding values of the parent species. d Paa, Pbb, Pcc : planar moments of inertia; definition is given in the text. e Number of lines in the fit. f Rootmean-square deviation of the fit.

The rotational spectra of several water isotopologues, the experimental spectroscopic parameters of which are also summarized in Table 2, were also investigated, in the same conditions optimized for the normal species. The bideuterated species (3FPY-DOD) and the isotopologue with

18

O (3FPY-

H18OH) were generated by adding D2O or H218O directly to the system while the two monodeuterated species (3FPY-HOD and 3FPY-DOH) were prepared using a mixture of 1:1 H2O: D2O. In the tables and in the text the notation -HOH indicates the first and second water hydrogens corresponding respectively to H12 and H14 in Figure 2. A careful search for the rotational transitions originating from the 3FPY-W_2 conformer was also performed but without success. The calculated energy for this species is very close to that of the global minimum (see Table 1) and sizable values of the dipole moment components are predicted, so that the rotational transitions of this conformer should be intense enough to be observed in the recorded spectra. The fact that it wasn’t observed is probably related to a relaxation process occurring in the jet due to a low interconversion barrier to the 3FPY-W_1 conformation.26 4.3 Structure Straightforward information on the structure can be derived from the experimental values of the planar moments of inertia, in particular the value of Pcc (Pcc = Σ mic2i = h/(16π2)(1/A + 1/B - 1/C)), which is a measure of the mass distribution out of the ab-plane. The experimentally determined values for Pcc are reported in Table 2 and for the normal, -H18OH and –DOH species these values


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are positive and very close to zero (0.07, 0.06 and 0.07 uÅ2 respectively), as expected for a planar system. The small positive contribution is generally compatible with the out of plane vibrations and, in this particular case, this is to be imputed mostly to a large amplitude motion of the free OH group of the weakly bound water. In fact, the substitution of the hydrogen atom not involved in the intermolecular interaction (H14) causes an increment of the Pcc values (0.23 and 0.25 uÅ2 for -HOD and DOD, respectively) consistent with the vibrational contribution of out of plane motions to the Pcc moment of inertia. The overall planarity is also in accord with the observation of µa and µb, but no µc-type transitions. Looking more closely at the experimental results and comparing the data contained in Table 1 and 2, one can see that the experimental rotational and quadrupole coupling constants are in good agreement with the calculated ones. It is worth noting that the theoretical structure is not planar (Pcc = 0.5 uÅ2), as reported also for the diazines-water systems. Since, as discussed above, the Pcc increment, upon H14→D14 substitution is to be imputed its implication in a large amplitude vibrational motion through the aromatic plane, the structural analysis of the complex was performed assuming planarity of the equilibrium structure. The same considerations were done also for the diazines and fluorobenzenes water complexes.9-12,17



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Figure 2. Structure of 3FPY-W_1 showing the principal axis system, the atom numbering and the intermolecular structural parameters used in the text.

We determined a partial experimental rs-structure using Kraitchman’s substitution method27 with Costain’s error procedure.28

Such analysis leads to the determination of the principal axis

coordinates of the substituted atom from the changes in principal moments of inertia resulting from a single isotopic substitution. The obtained water atoms coordinates are reported in Table 3 and compared with the calculated ones (re) and the ones obtained from the r0 structure (see below). Table 3. Principal axis coordinates for 3FPY-W_1. Substitution coordinates (rs) are compared to the MP2/6-311++G(d,p) data (re) and to those derived from the effective structure (r0). rsa O13

H12

H14 a

re

r0 a

a/Å

3.8330(4)b

3.812

3.836

b/Å

0.485(3)

-0.522

-0.502

a/Å

2.7395(6)

2.866

2.888

b/Å

0.895(2)

-0.715

-0.706

a/Å

4.2857(4)

4.238

4.267

b/Å

1.103(1)

-1.132

-1.360

c-coordinates are fixed to zero. b Constain’s errors28 expressed in units of the last decimal digit.

To extract information on the relative orientation of the two moieties, a partial r0-structural fitting was performed starting from the r0 structure of 3FPY,4 and adjusting the intermolecular parameters depicted in Figure 2; the H12N1C2 angle (φ) was fixed to the ab initio value, the H14O13H12N1 dihedral angle was constrained to 180°, while the H···N intermolecular bond distance (r) and OH···N hydrogen bond angle (θ) were adjusted to reproduce the observed rotational constants. In the r0 analysis we did not take into consideration the experimental rotational constants of the deuterated species because: (i) the Ubbelohde effect29 alters the O···N distance upon H→D substitution involved in the hydrogen bond; (ii) the free water hydrogen is implicated in a large amplitude motion. Nevertheless using the experimental rotational constants of the parent and

18

O

species, the r (r=1.9961(5) Å) and θ (θ =156.8(1)°) values could be determined. The STRFIT ACS Paragon Plus Environment

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program30 was used and the experimental rotational constants were reproduced within a few MHz. The starting geometry is available as Supporting Information. The results are compared in Table 4 to the theoretical ones and to those reported in the rotational spectroscopy studies of diazines-water complexes.9-12 Regarding the diazines, it can be observed that the length of the hydrogen bond decreases when the distance between the two heterocyclic nitrogen atoms increases. The value obtained for 3FPY-W_1 is close to the one determined for the pyrimidine-water adduct and this fact demonstrates that the insertion of a nitrogen atom on the ring or substitution with a fluorine atom in the same meta position has a similar effect on the hydrogen bonding properties of PY. It is difficult to find a rationalization for the value of the hydrogen bond angle, which is smaller than that found in pyrimidine-water, but it could be related to concerted action of mesomeric and inductive effect of the fluorine atom which might increase the positive charge on the C-H involved in the secondary interaction. Table 4. Comparison of the intermolecular structural parameters among aromatic amines-water complexes.

a

3FPY-W_1 (r0) pyrazine-water9 pyrimidine-water10 pyridazine-water11 a

r/Å 1.9961(5)b 1.94 1.98 2.04

θ/° 156.8(1) 152 167 166

φ/° [139.3]c 122 147 149

The corresponding MP2/6-311++G(d,p) calculated values are: r =1.9785 Å, θ =157.5°, φ =139.3°.b Error expressed in units of the last decimal digit. c Fixed to the re value.

4.4 Dissociation Energy The intermolecular stretching motion can be, in a first approximation, well isolated from the other low-frequency motions. For an asymmetric top complex in which the stretching motion between the two subunits is almost parallel to the a-axis of the complex, the stretching force constant (ks) can be roughly estimated using a pseudo-diatomic approximation through the following equation:31 ks = 16π4(µRCM)2[4B4 + 4C4 -(B - C)2(B + C)2]/(hDJ)


(1)


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where µ is the pseudo diatomic reduced mass, RCM (4.563 Å) is the r0 distance between the centers of mass (CM) of the two subunits, and B, C and DJ are the experimental rotational and distortion constants reported in Table 2. The value obtained for 3FPY-W_1 is ks = 17.5 Nm-1. Since this value is greater than the ones calculated for pyrimidine-water (ks = 13.8 Nm-1)10 and pyridazine-water (ks = 13.1 Nm-1)11, the O-H···N intermolecular hydrogen bond in 3FPY-W_1 adduct, turns out to be a stronger stabilizing interaction. The dissociation energy (De) of the 3FPY-W_1 complex was evaluated to be 30.5 kJ mol-1 by assuming a Lennard–Jones potential function, according to: De=1/72ksRCM2.32 This value is comparable to the theoretical results of Table 1 and considerably higher than the estimated binding energies of the diazines-water (about 20 kJ mol-1) complexes.9-12 Conclusions The analysis of the rotational spectra of the five isotopologues of 3FPY-W complex represents one of the first high resolution spectroscopy works about the microsolvation effect on fluorinated azines. The rotational constants and fourth order centrifugal distortion constants for the most stable conformer were obtained. Partial r0 and rs structures were determined from the rotational constants, and information about the orientation of the water subunit with respect to the 3FPY ring was obtained. Water approaches the PY ring with an overall planar arrangement, creating a main intermolecular O-H···N hydrogen bond (1.996 Å) and a secondary O···H11-C6 interaction (2.829 Å). Although other conformations characterized by a O-H···F linkage are found to be local minima, they were not detected in the rotational spectrum since the binding energy to the fluorine atom is estimated to be about 10 kJ mol-1 lower than that to the nitrogen lone pair. This difference in stabilization energies for the different conformers of 3FPY-W is in agreement with the values of the pseudo-diatomic stretching force constants determined in the water complexes of fluorobenzenes (ca. 5 kJ mol-1)17 and diazines (ca. 13 kJ mol-1),10-12 in which only a kind of proton-acceptor site is available. In conclusion the dissociation energy of 3FPY-W appears slightly higher than the values estimated for bare diazines and since the fluorine atom is not directly involved in the intermolecular ACS Paragon Plus Environment

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interaction this must be ascribed to the concerted action of mesomeric or inductive effect of fluorine substitution.

Acknowledgment. We thank the Italian MIUR (PRIN project 2010ERFKXL_001) and the University of Bologna (RFO) for financial support. Supporting Information Available: Full reference [23], MP2/6-311++G(d,p), starting geometry of 3FPY-W_1 used in the structural fitting procedure, and Tables of transition frequencies of all observed isotopic species. This information is available free of charge via the Internet at http://pubs.acs.org. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally.



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