Article pubs.acs.org/JPCA
Interactions between Carboxylic Acids and Aldehydes: A Rotational Study of HCOOH−CH2O Qian Gou,† Laura B. Favero,‡ Somana S. Bahamyirou,† Zhining Xia,§ and Walther Caminati*,† †
Dipartimento di Chimica “G. Ciamician” dell’Università, via Selmi 2, I-40126 Bologna, Italy Istituto per lo Studio dei Materiali Nanostrutturati (ISMN), Sezione di Bologna CNR, via Gobetti 101, I-40129 Bologna, Italy § Chemistry and Chemistry Engineering College, Chongqing University, Shazheng str., 174, 400040, Chongqing, P. R. China ‡
S Supporting Information *
ABSTRACT: The rotational spectrum of the 1:1 complex between formic acid and formaldehyde shows that the two units are linked together through a “classical” (OH···O) and a weak (CH···O) hydrogen bond. The molecular system appears quite rigid, and no effects of the internal motions have been observed in the spectrum.
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INTRODUCTION
that of H2O, indicating that CH2O undergoes a less hindered internal motion. To shed light on the nature of the internal motions of adducts between carboxylic acids and aldehydes, we decided to investigate the simplest of this kind of adduct, HCOOH− CH2O. No adducts between carboxylic acids and aldehydes have been rotationally characterized so far.
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Several molecular complexes involving carboxylic acids or aldehydes22−31 have been investigated by rotational spectroscopy to understand the nature of their noncovalent interactions and to have information on their internal dynamics and on their conformational equilibria. Most attention has been paid to the complexes of carboxylic acids, mainly to their dimers1−14 and to their adducts with water.15−21 Plenty of data have been obtained on the dimers, concerning proton tunneling, Ubbelohde effect, and conformational equilibria. As for the aldehydes, fewer rotational data are available, but the typology of investigated complexes is broader, ranging from van der Waals complexes with rare gases22−24 to dimers,25 and to adducts with water,26 CO2,27 halogens,28 hydrocarbons,29 or freons.30,31 In a number of hydrated carboxylic acids, small splittings, plausibly arising from an internal motion of water, have generally been observed. However, the tunneling motion generating these splittings has not yet been satisfactorily interpreted. 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. However, how this motion can cause a relatively complicated splitting pattern remains unclear.21 When replacing H2O with formaldehyde (CH2O), a molecule with the same symmetry (C2v), in the complexes with CH2F230 and CH2FCl,31 CH2O was found to form similar linkages with freons, undergoing the same kind of internal motions of H2O.32,33 Moreover, the observed tunneling splitting due to the internal rotation of CH2O is larger than © XXXX American Chemical Society
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EXPERIMENTAL SECTION
Commercial samples of HCOOH and paraformaldehyde 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-type34 pulsed supersonic-jet Fourier-transform microwave (FTMW) spectrometer,35 described elsewhere.36 Helium at a total pressure of 0.3 MPa was streamed over paraformaldehyde (heated to ∼350 K) and then over HCOOH at room temperature. The obtained mixture was expanded through the solenoid valve (General Valve, Series 9, nozzle diameter 0.5 mm) into the Fabry−Pérot-type cavity. Because of the coaxial arrangement of the molecular beam expansion and the resonator axes, each rotational transition displays an enhanced 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, and resolution is better than 7 kHz. Received: July 2, 2014 Revised: September 16, 2014
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dx.doi.org/10.1021/jp506600p | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
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Article
THEORETICAL CALCULATIONS To have a clue on the shapes and relative energies of the plausible isomers of HCOOH−H2CO, we performed geometry optimizations at the MP2/6-311++G(d,p) level of theory with the Gaussian 03 program.37 Two stationary points were found. The corresponding shapes, relative energies, rotational constants, and dipole moment components are reported in Table 1. In isomer I, one “classical” O−H···O and one weak
abundance. These spectra were so weak that we could only measure the eight most intense transitions for each of them. From these eight transitions it was not possible to determine the centrifugal distortion constants that were 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. Finally, we assigned and measured the spectra of the DCOOH−H2CO and HCOOD−H2CO monodeuterated isotopologues. The first one was obtained using 98% enriched DCOOH (Aldrich), while the second one was prepared by direct exchange with D2O. For these two species we observed the quadrupolar effect due to the D atoms (which have nuclear spin quantum number I = 1). To analyze the corresponding hyperfine structure of the rotational transitions, we used Pickett’s SPFIT program.40 The fitted parameters are shown in the two right columns of Table 2. All diagonal D-quadrupole coupling constants have been determined. We report in Table 2 also the inertial defects, Δc, of the five isotopologues. Their values are very close to zero, suggesting a planar arrangement of the complex. As for the changes in the rotational constants upon OH → OD deuteration, they are not so marked as in the cases of very large complexes (see, for example refs 10 and13), so it is not straightforward to extract information on the Ubbelohde effect.41 Structural Information. In the observed conformer, shown in Figure 1, CH2O is linked to HCOOH through a main O− H···O HB, while a C−H···O WHB also contributes to the stability of the complex.
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−CH2O
a
Absolute values are −303.618088 and −303.615326 Eh, respectively.
C−H···O hydrogen bond (HBs) link the two subunits together, while isomer II displays two weak C−H···O interactions (weak hydrogen bond, WHB). This is reflected in the quite higher energy of form II. To have a better estimate of the energy differences, both intermolecular binding energy values were counterpoisecorrected for basis-set superposition error (BSSE),38 giving also the BSSE-corrected dissociation energies (ED‑BSSE). Rotational Spectrum. Guided by the ab initio calculation previously presented, we assigned first the J: 2 ← 1 a-type band transitions for isomer I. Later on several a- and b-type transitions have been assigned and measured for at total number of 16 lines. All transitions have been fitted within Irrepresentation of Waltson’s S reduction.39 Rotational and the five 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
Figure 1. Sketch, atom numbering, main structural parameters, and principal axes of the observed form of HCOOH···CH2O.
Table 2. Experimental Spectroscopic Constants of the Observed Isotopologues of HCOOH−CH2O (S-Reduction, Ir Representation) HCO2H−CH2O A/MHz B/MHz C/MHz DJ/kHz DJK/kHz DK/kHz d1/kHz d2/kHz χaa/kHz (χbb − χcc)/kHz Δc/uÅ2c σ/kHzd Ne
9365.030(3) 2742.751(2) 2121.499(1) 2.57(4) 14.2(3) −9.7(7) −0.68(3) −0.14(2)
−0.006(1) 0.4 16
a
HCO2H−13CH2O
H13CO2H−CH2O
DCO2H−CH2O
HCO2D-CH2O
9307.692(3) 2683.302(2) 2082.899(1) [2.57]b [14.2] [−9.7] [−0.68] [−0.14]
9364.985(3) 2703.854(2) 2098.166(1) [2.57] [14.2] [−9.7] [−0.68] [−0.14]
−0.007(1) 3.2 8
−0.001(1) 2.6 8
9361.102(3) 2637.299(2) 2057.911(1) 2.32(4) 12.7(3) [−9.7] −0.59(3) [−0.14] 160(4) 11(2) −0.036(1) 2.2 51
9211.714(3) 2742.369(2) 2113.342(1) 2.49(5) 14.5(5) [−9.7] −0.65(4) [−0.14] 203(3) 38(2) −0.011(1) 2.9 39
a Error in parentheses in units of the last digit. bValues in brackets fixed to the values of the most abundant isotopic species. cInertial defect. dRootmean-square deviation of the fit. eNumber of lines in the fit.
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dx.doi.org/10.1021/jp506600p | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
From the rotational constants of the five isotopologues, it is possible to calculate the substitution coordinates, rs, of the four substituted atoms in the principal axes of the parent species.42 The obtained data are summarized in Table 3 where the values
where μD is the pseudodiatomic 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 = 23.9 N m−1, corresponding to a harmonic stretch fundamental ν = 149 cm−1. Assuming a Lennard-Jones type potential, the zero-point dissociation energy of the complex can be estimated to be 19.2 kJ mol−1 by applying the approximate expression44
Table 3. Experimental (rs and r0) and Ab Initio (re, MP2/6311++G(d,p) Coordinates of the Substituted Atoms of HCOOH···CH2O (See Figure 1) C1 |a|/Å
|b|/Å
a
rs r0c re rs r0 re
1.6361(9) 1.6399 1.6487 0.02(9) 0.04 0.06
b
C6
H4
H5
2.0278(7) 2.0579 2.0708 0.588(3) 0.592 0.580
2.7238(5) 2.7303 2.7368 0.16(1) 0.16 0.08
0.166(9) 0.017 0.062 0.951(1) 0.911 0.947
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Coordinate c set to zero by symmetry. bError in parentheses are in units of the last digits. cCalculated from the partial r0 structure in Table 4.
of a partial r0 structure and of the ab initio (re) structure are also given for comparison. c coordinates have been set to zero by symmetry. The most remarkable discrepancy is on the a coordinate of the acid hydrogen involved in the HB, as usual for protons involved in hydrogen bonds and for values of coordinates close to zero. The partial r0 structure has been obtained from the available rotational constants of all isotopologues except those of the OD species, whose values are not reliable for structure determination because of the Ubbelohde effect. Five structural parameters have been adjusted to fit 12 rotational constants while keeping the remaining parameters fixed to their ab initio values. After the structure fitting, the largest discrepancy between the experimental and the r0 calculated rotational constants is
