Shape of the Adduct Formic AcidâDimethyl Ether: A Rotational Study

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Shape of the Adduct Formic Acid-Dimethylether: A Rotational Study Luca Evangelisti, Lorenzo Spada, Weixing Li, Anna Ciurlini, Jens-Uwe Grabow, and Walther Caminati J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b02912 • Publication Date (Web): 22 Apr 2016 Downloaded from http://pubs.acs.org on April 23, 2016

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The Journal of Physical Chemistry

Shape of the Adduct Formic Acid-Dimethylether: A Rotational Study. Luca Evangelisti,a Lorenzo Spada,a Weixing Li,a Anna Ciurlini,a Jens-Uwe Grabow,b Walther Caminati a,* a

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

b

Institut für Physikalische Chemie und Elektrochemie, Lehrgebiet A, Gottfried-Wilhelm-

Leibniz-Universität, Callinstrasse 3A, D-30167 Hannover, Germany

Corresponding Author *Walther Caminati. Email: [email protected]; Ph.: +39-0512099480

ACS Paragon Plus Environment

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ABSTRACT. Formic acid and dimethylether are combined in a supersonic expansion to form a molecular adduct with the two subunits held together by a “classical” OH···O hydrogen bond and a bifurcated weak CH2···O hydrogen bond. The rotational spectra of the parent and of two 13

C isotopologues in natural abundance show that the complex has Cs symmetry, with the heavy

atoms symmetry planes of HCOOH and (CH3)2O perpendicular to each other.


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1. Introduction The investigation of molecular adducts by high resolution spectroscopy supply information on the non-covalent interactions which link together the constituent molecules, and which are of importance in determining, for example, the shapes of biomolecular aggregates. The energies of various kinds of interactions have been and can be determined. It has been established a ranking among the strengths of several interactions and it has been observed that some times hydrogen bond can be overwhelmed by halogen bond or by n→π* linkages. Occasionally tunneling splittings have been measured that have been used to determine potential energy surfaces. Among these complexes, recently considerable efforts have been dedicated to the characterization of the properties of molecular adducts involving carboxylic acids by rotational spectroscopy. Most of these complexes are dimers constituted by two carboxylic acid units. Although the first observation of such a dimer – by low resolution microwave spectroscopy dates to several decades ago,1,2 the most detailed information on phenomena like proton transfer, other internal motions, conformational equilibria and Ubbelohde effect have been obtained only recently.3-15 High resolution results on the dimers have been obtained for the homodimer of benzoic acid by rotationally resolved LIF spectroscopy.16 High-resolution infrared spectroscopy has been dedicated to the study of the non-polar dimer of formic acid.17 Rotational spectra are also reported on a number of hydrated carboxylic acids.18-23 These adducts display small splittings, plausibly arising from an internal motion of water, that have generally been observed. Jäger et al. hypothesized four different pathways in the case of benzoic acid-water and pointed out that the rotation of water about the lone pair hydrogen-bonded to benzoic acid is likely the only candidate for the observed splittings with four components.23


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Finally, the complexes of formic acid with partner molecules containing various functional groups, like anhydrides,24 aldehydes,25 amides26 and azines27 have been characterized by rotational spectroscopy. Two of these partner molecules (formamide26 and pyridine27) form with formic acid a relatively strong linkage (~ 40 kJ/mol), based on a planar “near-aromatic” sevenmembered ring, which includes two hydrogen bonds. CO224 and CH2O25, however, exhibit an interaction energy with HCOOH, which is only about half (~ 20 kJ/mol) of that observed for formamide and pyridine. No data are available on adducts of carboxylic acids with ethers. What will be the shape and the interaction energy of the molecular adduct of formic acid and dimethylether ((CH3)2O)? This microwave spectroscopic study aims to answer this question qualitatively and quantitatively.

2. Experimental Section Commercial samples of HCOOH and (CH3)2O were obtained from Aldrich and used without further purification. The rotational spectra in the 6-18.5 GHz frequency region were measured on a COBRA-type28 pulsed supersonic-jet Fourier-transform microwave (FTMW) spectrometer,29 described elsewhere.30 A mixture of (CH3)2O with ca. 1% in Helium at a total pressure of 0.3 MPa was streamed over HCOOH at 273 K and then expanded through the solenoid valve (General Valve, Series 9, nozzle diameter 0.5 mm) into the Fabry-Pérot-type cavity. Due to the coaxial arrangement of the molecular jet expansion and the resonator axes, each rotational transition displays a pronounced Doppler splitting. 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.


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3. Theoretical calculations In order to get a first estimate on the shapes and relative energies of the plausible isomers of HCOOH-(CH3)2O, we performed geometry optimizations at the MP2/6-311++G(d,p) level of theory with the Gaussian 09 program.31 Two stationary points were found. The corresponding shapes, relative energies, rotational constants, and dipole moment components are reported in Table 1. In the most stable isomer (O), the heavy atoms symmetry planes of the two constituent molecules are orthogonal to each other, while in the second one (NCP) the symmetry planes are nearly coplanar. The two constituent molecules are held together by one “classical” O–H···O hydrogen bond (HB) and a bifurcated weak CH2···O hydrogen bond (WHB) in species O, and again by one OH···O interaction but only a simple weak C–H···O hydrogen bond in species NCP. In order to have a better understanding of the molecular properties, the nature of all stationary points was verified by subsequent harmonic frequency calculation. Estimates of the intermolecular binding energy values for the most stable species were counterpoise corrected for basis set superposition error (BSSE)32 also applying the zero-point energies (Eo). Table 1. MP2/6-311++G(d,p) shapes, relative energies, spectroscopic constants and dipole moment components of the two most stable forms of HCOOH-(CH3)2O. O

NCP

Relative energy

∆E, ∆E0 /kJ mol-1

0a , 0b

2.3, 1.4

Dissociation energy

ED, ED,0/kJ⋅mol-1

47.5, 40.9 33.5, 27.0

-


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ED,BSSE, ED,0,BSSE/kJ⋅mol-1 A, B, C/MHz

Rotational constants 5160, 1858, 1834c 5955, 1684, 1341 Electric dipole 1.7, 0.1, 0.0 2.8, 0.5, 0.5 µ , µ , µ /D moment components a b c a Absolute energy: -344.0043087 Eh. bAbsolute energy: -343.8865972 Eh cFor conformer O also quartic centrifugal distortion constants have been calculated: DJ = 1.78, DJK = 8.23, DK = -7.16, d1 = -0.030, d2 = 1.31 kHz, respectively.

4. Results 4.1 Rotational spectrum. The ab initio calculation suggested the O isomer to be the most stable species and µa (= 1.7 D) to be its strongest dipole moment component. For this reason, we started our spectral search with the J 2←1 µa-type R band. We assigned all three transitions of this band, 202←101, 212←111 and 211←110. It has then been easy to measure all a-type transitions falling in our spectral range, for at total number of 22 lines. We were not able to detect any b- or c-type line, according to the very low calculated µb value and µc being zero by symmetry. All µatransitions have been fitted within the Ir-representation of Waltson’s S reduction.33 Rotational and four quartic centrifugal distortion constants have been precisely determined and are shown in the left column of Table 2. After having assigned the spectrum of the parent species, we could assign the spectra of the two mono-13C species in natural abundance. These spectra were very weak (1% and 2% for H13CO2H-(CH3)2O and for HCO2H -13CH3OCH3, respectively), therefore we measured only the 10 most intense transitions for each species. From these 10 transitions the centrifugal distortion constants were undetermined and fixed at the values of the corresponding parent species parameters. The results of the fits are reported in the second and third columns of Table 2. Table 2. Experimental spectroscopic constants of the observed isotopologues of HCOOH(CH3)2O (S-reduction, Ir representation)


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HCO2H -(CH3)2O

HCO2H 13 - CH3OCH3

H13CO2H -(CH3)2O

HCOOD -(CH3)2O

DCOOH -(CH3)2O a

A/MHz 5168(8)b 5095c 5168c 5126c 5167c B/MHz 1824.846(2) 1799.4138(6) 1800.2425(6) 1823.1162(5) 1766.2896(6) C/MHz 1808.391(2) 1790.7518(6) 1784.2554(6) 1801.1519(5) 1750.8267(6) DJ/kHz 2.08(1) [2.08]d [2.08] [2.08] 1.87(1) DJK/kHz 6.78(4) [6.78] [6.78] [6.78] [6.78] -0.05(2) [-0.05] [-0.05] [-0.05] [-0.05] d1/kHz 0.190(7) [0.190] [0.190] [0.190] [0.190] d2/kHz e 6.4 8.1 7.1 5.9 2.2 σ /kHz f 16 10 10 18 36 N a b D nuclear quadrupole coupling constant χaa = 0.37(3) MHz. Error in parentheses in units of the last digit. cExtrapolated from the experimental A value of the parent species by applying the ab initio predicted difference between isotopologues. dValues in brackets fixed to the corresponding values of the parent species. eRoot-mean-square deviation of the fit. fNumber of lines in the fit. Finally we assigned and measured the spectra of the DCOOH-(CH3)2O and HCOOD-(CH3)2O mono-deuterated isotopologues. The first one was obtained using 98% enriched DCOOH (Aldrich), while the second one was prepared by direct exchange with D2O. For the last species we observed the quadrupolar effect due to the D nucleus (nuclear spin I = 1). In order to analyze the corresponding hyperfine structure (hfs) of the rotational transitions, we used Pickett’s SPFIT program.34 We could determine only χaa (= 0.37(3) MHz), while χbb-χcc was fixed to zero, according to the very small value obtained from the ab initio calculations. In Figure 1 we compare the 211←110 transition of the parent species to that of the DCOOH-(CH3)2O isotopologue. In the latter case the transition is split in four D-hfs component lines.


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Figure 1. Transition 211←110 of formic acid and dimethylether: (a) HCOOH-(CH3)2O parent species (270 FIDs). (b) DCOOH-(CH3)2O isotopologue (1000 FIDs); the four D-hfs components F’←F” are labeled with the total angular momentum quantum number F = | J + I |. Rotational constants A of the less abundant species have been fixed to the values extrapolated from A of the parent species: the difference between the experimental and model (see next section) calculated values of A of the parent species has been applied to the model values of all less abundant isotopologues.

4.2 Structural Information. The observed conformer, shown in Figure 2, has a plane of symmetry, containing atoms 1-6 and the reference point X7 (lying along the bisector of the C8O6C9 angle). (CH3)2O is linked to HCOOH through a main O-H···O HB, and a bifurcated CH···O WHB.


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Figure 2. Sketch, atom numbering, main structural parameters and principal axes of the observed form of HCOOH···(CH3)2O. Number 7 indicates a dummy atom (0 mass) placed on the bisector of the C8O6C9 angle. The corresponding MP2/6-311++G(d,p) geometry is reported in Table 3. Table 3. MP2/6-311++G(d,p) geometry of the observed isomer of HCOOH···(CH3)2O. Bond lengths/Å

Valence Angles/˚

Dihedral angles/˚

O2C1 1.2140 O3C1 1.3321 O3C1O2 125.7 H4C1 1.0982 H4C1O2 123.7 H4C1-O2O3 180.0 H5O3 0.9939 H5O3C1 106.9 H5O3-C1O2 0.0 O6H5 1.6692 O6H5O3 179.9 O6H5-O3C1 0.0 X7O6a 1.0000 X7O6H5 128.0 X7O6-H5O3 0.0 C8O6 1.4245 C8O6X7 55.4 C8O6-X7H5 90.0 C9O6 1.4245 C9O6X7 55.4 C9O6-X7H5 -90.0 H10C8 1.0904 H10C8O6 107.0 H10C8-O6C9 180.0 H11C9 1.0904 H11C9O6 107.0 H11C9-O6C8 180.0 H12C8 1.0960 H12C8O6 110.1 H12C8-O6H10 -118.9 H13C9 1.0960 H13C9O6 110.1 H13C9-O6H11 118.9 H14C8 1.0960 H14C8O6 110.4 H14C8-O6H10 119.5 H15C9 1.0960 H15C9O6 110.4 H15C9-O6H11 -119.5 a X7 indicates a dummy atom (0 mass) placed on the bisector of the C8O6C9 angle. Since the error on the rotational constant A is – due to the missing of perpendicular transitions relatively large, we could not obtain precise values of the substitution coordinates.35 However,


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we could find an effective (partial r0) structure, able to reproduce the experimental rotational constants. Taking the ab initio geometry as a starting point, and just applying a correction to the O6···H5 hydrogen bond length (from 1.669 to 1.673(2) Å), it has been possible to reproduce the rotational constants B and C of all isotopologues within 1-2 MHz, and the rotational constant A within its experimental uncertainty. The hydrogen bond structural parameters obtained with this effective geometry are given in Table 4. Table 4. r0 structural parameters of the hydrogen bonds which link the constituent molecules of HCOOH···(CH3)2O. O-H···O HB rO6H5/Å ∠O6H5O3/° ∠X7O6H5/°

C-H···O WHB

1.673 179.9 128.0

rO2H14(15)/Å ∠C8H14(15)O2/° ∠C1O2H14(15)/°

2.824 115.5 112.5

In Table 5 we compare the experimental and ab initio values of the planar moment of inertia Pcc. It is defined as Pcc = 1/2(Ia+Ib-Ic) and it gives the mass extension along the c-axis, i.e. the mass extension out of the ab-plane. For the O isomer, it should correspond to the Paa value of isolated (CH3)2O,36 which is also reported in Table 5. Table 5. Experimental and ab initio Pcc values of HCOOH···(CH3)2O are compared to the Paa value of (CH3)2O. Pcc(HCOOH-O(CH3)2)/uÅ2 Exptl.

ab initio

Paa(O(CH3)2)/uÅ2 Exptl.

47.6(2)a 47.2 47.0466(1)b a Error in parentheses in units of the last digit. bFrom Ref. 36.


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Both experimental and ab initio Pcc values of HCOOH-O(CH3)2 are larger than the Paa value of (CH3)2O. It seems that the COC dihedral angle of (CH3)2O becomes slightly larger upon complexation with HCOOH.

4.3. Information on the dissociation energy. The intermolecular stretching motion which leads to the dissociation is almost parallel to the a-axis of the complex. In this case, a pseudo diatomic approximation can be used to roughly estimate the force constant of the stretching mode leading to the dissociation through the equation:37 ks = 16π 4 (µ D RCM )2 [4 B 4 + 4C 4 − ( B − C )2 × ( B + C )2 ] /(hDJ )

(1)

where µD is the pseudo diatomic reduced mass, RCM (= 3.1 Å) is the distance between the centers of the mass of the two subunits, and B, C and DJ are the spectroscopic parameters of Table 2. We obtained ks = 21.4 N m-1, corresponding to a harmonic stretch fundamental ν = 142 cm-1. Assuming a Lennard-Jones type potential the zero point dissociation energy of the complex can be estimated to be ~11 kJ mol-1 by applying the approximate expression:38 2 EB = ks RCM / 72

(2)

Such a value is considerably lower than the ab initio suggestions. We found the pseudo diatomic approximation to work and to give consistent results for several families of related adducts. However, in the present case we are unhappy with the result. Probably, for HCOOHO(CH3)2 the detachment motion does not take place along the a-axis (for example, the carbonyl group, which is linked to O(CH3)2 by a WHB, could move away before of the OH group). Then, the stretching motion could be coupled to bending vibrations making the pseudo diatomic approximation to crude.

5. Conclusions


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We reported the results of the first rotational investigation of a molecular adduct constituted of a carboxylic acid and an ether. We observed the spectrum of the most stable isomer (O) and described the structural aspects of the non bonding interactions which link the two subunits, a classical OH···O HB and a bifurcated CH···O WHB. The second more stable (NCP) was not observed, probably because undergoing a conformational relaxation to O upon supersonic expansion.39 The dissociation energy appears comparable to the values obtained for HCOOH-CO224 and HCOOH-CH2O,25 and about half the values reported for the complexes of HCOOH with formamide26 and pyridine,27 where a quasi-conjugated ring system is formed.

ASSOCIATED CONTENT Supporting Information. 1) Completion of Reference 36; 2) Table with MP2/6-311++G(d,p) optimized geometry of the complex; 3) Two tables of transition frequencies. This material 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. ACKNOWLEDGMENT We thank the Italian MIUR (PRIN project 2010ERFKXL_001) and the University of Bologna (RFO) for financial support. L.E. was supported by Marie Curie fellowship PIOF-GA-2012328405. W.L. thanks the China Scholarships Council (CSC) for financial support.


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Publication

Date

(Web):

February

17,

2016

(Article).

DOI:

10.1021/acs.jpca.6b00387. 28)

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Shape of the Adduct Formic AcidâDimethyl Ether: A Rotational Study

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