J . Phys. Chem. 1993,97, 10925-10936
10925
Matrix Isolation Infrared and ab Initio Studies on Conformers of Fluoroacetic and Chloroacetic Acid J. Nieminen,' M. Pettersson, and M. Riisanen Department of Physical Chemistry. P.O. Box 13 (Meritullinkatu 1 C), SF-00014 University of Helsinki, Finland
Received: May 18, 1993; In Final Form: August 10, 1993'
The infrared spectra of matrix isolated fluoroacetic acid and chloroacetic acid were studied in solid Ar, Kr, and Xe matrices. The energies and spectra of possible conformers of fluoroacetic acid and chloroacetic acid were simulated by extensive ab initio calculations at the MP2/6-3 11G** (energies, geometries), MP2/6-3 1G** (barriers), and MP2/4-3 lG* (numerical frequencies) levels. The present calculations propose the existence of four distinct conformers. For fluoroacetic acid the two lowest energy conformers, i.e., Tt and Ct, were found to be present after deposition of the solids. These conformers differ by 180' rotation of the fluoromethyl group and for both conformers the carboxylic hydrogen atom is in cis position with respect to the C = O bond. In agreement with the experimental results, the ab initio calculated energy difference between Tt and Ct is ca. 1 kJ mol-', and the barrier for Tt Ct interconversion is ca. 13 kJ mol-'. For chloroacetic acid three conformers were identified in the matrix spectra. The lowest energy form Tt and C, symmetry. The second lowest energy conformer Gt differs from Tt by rotation of the chloromethyl group about the C-C bond (dihedral angle C1-C-C-0 = 7 7 O ) . The observed data and the calculations predict consistently a barrier of ca. 5 kJ mol-1 for this interconversion. For both conformers the carboxylic hydrogen atom is in cis position with respect to the C=O bond. The third observed conformer is Cc with intramolecular hydrogen bonding C1-H. A normal coordinate analysis based on the ab initio calculated force field is presented for observed conformers and compared with the experimental spectra. The similarities with related molecules are discussed.
-
I. Introduction l-Fluoroacetic acid (FAA) and l-chloroacetic acid (CAA) are the simplest molecules in series of a-substituted carboxylic acid. According to previous e~perimentall-~ and theoreticalstudies, both acids have at least two distinct conformers. The matrix isolation technique combined with FTIR spectroscopy provides very efficient tools to study energetic of these ground state conformers. It is known that the frequencies and intensities of fundamental absorptions depend on conformation of the molecule. The gas-phase spectra of molecules such as FAA are very rich due to rotations, and the assignment of conformers is difficult. On theother hand, thecorrespondingspectrumof matrix isolated species is much simpler due to lackof rotational structure. Comparison of matrix spectra of similar molecules also gives detailed information about the structural influences to spectrum. In the present case, comparison of spectra of FAA, CAA, and acetic acid gives informationof spectral and structural influences of halogen substitution of the methyl group. Furthermore, the observed spectral changes initiated either IR-photochemicallyor thermally, combined with ab initio calculations, give substantial data on conformationalenergeticsin the electronic ground state.9 According to our knowledge, no matrix isolation study on FAA has been done previously. Van Eijck et al. have studied the rotamers of FAA by electron diffraction and microwave techniques.1.2 They proved the existence of two conformersin the gas phase, labeled Tt and Cc in this paper. The upper-case letter designates the dihedral angle F-C-C-0 ( T , Trans; C, Cis) and the lower-case letter designatesthe dihedral angle H-O-C-C ( t , trans; c, cis). According to p r e v i o ~ sand ~ ~ present ~ ab initio calculations, the second lowest energy species for FAA is Ct. However, the estimated small dipole moment of rotamer Ct compared to Tt and Cc can make it difficult to find Ct by using microwave technique.* For CAA, three Trans conformers with respect to internal rotation around the C-C bond were found by combining the Abstract published in Aduunce ACS Abstracts, October 1, 1993.
0022-3654/93/2097- 10925$04.00/0
microwave and electron diffraction studies.3t4 In agreement with the previous ab initio calculation^,^ the lowest-energy conformer was found to be Tt having C, symmetry and C-Cl bond eclipsed with the C - 0 bond. For the two other conformers, with amounts of 30% and 14% at 170 OC, the CHzCl group was found to be rotated 131O and 79' from syn position? respectively. However, previous molecular mechanics study* indicates the existence of only two t conformers. In addition, Fausto et aL8 found two c conformers with dihedral angles C l C C 4 of Oo and 98O. A previous matrix isolation study of CAA indicates the presence of three conformers in Ar m a t r k 5 The author of that study suggests that there are two c and one t conformer with an internal OH4 1 hydrogen bond. The main motivation for studying IR spectra of matrix isolated FAA and CAA was to obtain information about energetics of conformers of these molecules. In section I1we report the spectral results in sold Ar, Kr, and Xe hosts. To aid the interpretation of these results, ab initio calculations were performed at various levels of theory. The highest level used for optimization of conformers was MP2/6-311GS*. The torsional potentials were simulated at level MP2/6-3 1**, and the harmonic frequencies for each conformer were calculated at level MP2/4-31G*. The ab initio results and potential energy distribution of calculated vibrational spectra are given in section 111. A comparison of experimental and theoretical results is discussed in section IV.
II. Experimental Section ExperimentalDetails. The IR spectra of FAA and CAA were studied in Ar, Kr, and Xe matrices. The spectra were measured with a Nicolet SX-60 FTIR spectrometer equipped with a (Air Products) HS-4 cryostat. The matrix samples were prepared by sweeping host gas over solid acid contained in a Pyrex tube. The host gas flow rate was controlled by a needlevalvesituated before the tube. The temperature of the tube was varied from -30 to 70 OC. Before use the acids were dried by vacuum sublimation. High-purity Ar, Kr, and Xe gases (>99.995%) were used. As 0 1993 American Chemical Society
10926 The Journal of Physical Chemistry, Vol. 97, No. 42, 1993
Nieminen et al.
TABLE I: Thermal and IR-Induced Changes in Absorption Bands (em-’) of Fluoroacetic Acid in Ar, Kr, and Xe Matrices.
1080
1280
1480
1680
3480
3520
3560
3600
Wavenumber (cm-’) Figure 1. At matrix spectra of FAA (upper trace) and CAA (lower trace), obtained after matrix deposition at 20 K. II
e
I
m
I\
0
3540
3550
3560
3570
Waven um ber (cm-l) Figure 2. Observed IR-induced proccss in the OH stretching region of fluoroacetic acid in Ar. Solid line: after deposition at 20 K. Dashed line: after 4 days of IR irradiation. previously mentioned, the dimeric species dominate the vapor phase of acetic acids.5 To decrease the extent of dimers, the gas mixture was led through a copper capillary heated to 100 OC, and this was found to be adequate to decrease the extent of dimerisation. The gas mixture was sprayed onto a cooled CsI window. The matrix-to-sample ratio could not be determined accurately, but mainly monomeric samples could be obtained by optimizing the substance and capillary temperature and the matrix-gas flow rate. (Also the associated forms can be identified by comparing matrices prepared under different conditions, for example without the heated copper capillary). For IR excitation we used the globar of the spectrometer. Results. Figure 1 shows typical survey spectra of FAA and CAA obtained after deposition of the Ar matrix at 20 K. For FAA there are no previous IR spectroscopic matrix isolation studies. For CAA there exists one previous Ar matrix study.5 Our Ar matrix spectrum of CAA is in a good agrement with the spectrum in that study. Fluoroacetic Acid. For FAA, the wavenumbers of absorption bands and their intensities are given in Table I. The number of the observed bands for monomeric FAA in the region 4000-400 cm-1 exceeds that which can be expected for only a single conformer. For example in the O H stretching region there are two strong bands at wavenumbers of 3566 and 3555 cm-1. In the v ( C q 0 ) region there are two strong bands at wavenumbers of 181 1 and 1779cm-1 and additionallyone much weaker absorption at 1804cm-l (see Figures 2 and 3). For acetic acid10 having only one conformer the wavenumbers of the corresponding bands in Ar are at 3566 and 1779 cm-1. After deposition, the structure of the FAA spectrum is very similar in Ar and Kr hosts, although somedifference due to matrix effects can be observed. For example, in Kr the strong v(0H) bands at 3548 and 3544 cm-l were redshifted by about 18 and 1 1 cm-1 from their values in Ar, respectively. In the C=O region the corresponding shifts were only 1-3 cm-1. In Xe we observed
Ar
Kr
3566 m 3555 m 1811 vs 1804 w 1779 m 1443 vw 1412 vw 1397 w 1369 vw 1357 vw 1328 w
3548 w 3544 w 1808 vs 1803 w 1789 1777 s 1440 vw 1410 vw 1396 w 1369 vw 1357 vw 1321 w
1304 vw 1283 vw
1306vw 1263 vw
1211 vw 1163 w 1136vw 1119vw 1112 1101 1092 m 1086 s
1208 vw 1159 vw 1136 vw 1119vw
1082 vw 1035 vw 979 vw
Xe 3523 vs
1806 vs 1804 w 1782 1778 m 1436 vw 1414 vw 1400 vw 1375 vw 1324 w 1321 vw 1306 vw 1274 1258 1216 1158 w 1127vw
941
I I1 111 I1 I11 I1 I
* * *
I1
I I
* I
1080 w 1032 vw
1035 vw
*
I1 I
*
1104 1098 1090 w 1087 s
1094 vw 1088 w 1085 s
IV I I1 I 111
I1 I11 I
* *
946 927 872 vw
862 vw I 860 vw 867 vw 111 857 vw 851 vw 861 vw I1 849 vw I1 649 w 647 vw 642 w I1 613 m 612 w 615 w I 599 w 596 w 596 w I1 534 vw 532 vw 535 vw 111 507 vw I1 492 w 491 vw I a I: decrease on IR excitation, increase on annealing. 11: decrease on annealing, increase on IR excitation. 111: slight change in IR excitation, no change in annealing. IV: no change; * associated form.
1775
1785
1795
1805
1815
Wavenumber (cm-’) Figure 3. Observed IR-induced process in CO stretching region of fluoroacetic acid spectrum in Ar host. Solid line: after deposition at 20 K. Dashed line: after -100 h of IR irradiation. only one v(0H) band at 3523 cm-l. However, the intensity of this band is about double that in Ar and Kr hosts when normalized with the intensities of the v(C=O) absorptions. The C=O stretching region in Xe is very similar to that in Ar and Kr. For FAA, the most intense absorptions in the region 400-4000 cm-1 are at 1086, 1085, and 1087 cm-l in Ar, Kr, and Xe hosts, respectively. Upon prolonged IR irradiation at 13 K, considerable changes occur in the spectra of Ar samples deposited at 20 K. During this process the absorption intensities of nearly half of the bands
Fluoroacetic and Chloroacetic Acid decrease while the rest gain intensity. In the u(0H) and v ( C 4 ) regions the intensity of the bands at 3566 and 1811 cm-1 decrease and the intensity of the bands at 3555 and 1779 cm-1 increase (see Figures 2 and 3). In addition there are a few very weak bands, such as that at 1804 cm-1, showing increase during IR irradiation. However, the relative change in these bands was considerably smaller than in the other bands. We use roman numericals 1-111 in Table I to label the bands belonging to these three categories. In argon we found 12 I bands decreasing during IR irradiation, 12 I1 bands with reverse changes, and 6 I11 bands with only minor IR-induced change. The process was similar in most of the bands in Kr and Xe hosts, although some differences were observed. In Xe the intensity of the strong band at 3523 cm-l did not change. The intensity of the very weak I11 bands increases slowly in Ar but decreases slowly in Kr and Xe (for example the bands at 1803 and 1804 cm-I in Kr and Xe, respectively). Besides IR photolysis without filters, the IR-induced process was observed also by using a band pass filter with transmitting between 2000 and 600 cm-I. Radiation below 1500 cm-l did not change the spectra during 18 h of IR excitation. A reverse thermal process to the photoprocess was observed in bands I and I1 during annealing. The onset of this process is at ca. 42 and 45 K in Kr and Xe, respectively. The intensities of I11 bands did not change during annealing. In Ar, no change was observed during annealing the sample before it was destroyed. In the u ( 0 H ) region (in Kr) the intensity of the band at 3548 cm-1 increases and the intensity of the band at 3544cm-1 decreases during annealing. In Xe no new bands were observed and the intensityof the band at 3523 cm-l did not change during annealing. In Kr at the u(C=O) region, the intensity of the 1808-cm-l band increases, the band at 1777 cm-1 almost disappears, and the intensity of the band at 1803 cm-1 was unchanged. The observed process was similar in the Xe host. IR excitation of the thermally treated sample initiated the same process as observed after deposition in Kr and Xe. Figures 4 and 5 display the process in Kr and Xe in the u ( C 4 ) region. As can be seen from Figure 4, the doublet structure for band at 1808 cm-l changes during this process in Kr. As will be later shown the best explanation for all these experimental results is the presence of at least two distinct conformers of FAA (1,II) in matrix isolated samples. By using this assumption and normalizing the absorption bands with IRinduced and thermal changes the energy difference between conformers I and I1 can be estimated. An experimental energy difference of the order 1 kJ mol-’ between conformers I and I1 is obtained assuming that the gas-phase equilibrium is trapped during deposition. Bands I11 can be excluded in the estimation of the energy difference between species I and I1 for the following reason: The initial intensity and change during IR excitation in I11 bands is very small compared to I and 11 bands, and there were no changes in I11 bands during annealing. Chloroacetic Acid. As for FAA, the number of monomeric absorption bands for CAA in the 4000-400-~m-~ region exceeds that expected for only one conformer. The wavenumbers of the observed bands in Ar, Kr, and Xe hosts are given in Table 11.The spectrum of CAA is very similar in Ar and Kr; however, the site structure is simpler in Kr than in Ar. For FAA, variation of the deposition temperature between 13 and 20 K (in Ar) did not cause any changes in the spectrum. However, considerable changes are observed for CAA. As previously noted,s if CAA/Ar is deposited at 13 K, new bands appear and the intensity of some bands increase. For example, in the u(C-0) and u(0H) regions the intensity of the bands at 1773 cm-1 and at 3560 cm-l increase compared to the bands at 1806and 3566cm-I. Redistributionof absorption bandintensities occurs if the sample deposited at 13 K was annealed at 18 K, in accordance with previous studies.5 However, annealing at least
The Journal of Physical Chemistry, Vol. 97, No. 42, 1993 10927
I
I1
Q
2
(d
e k 0
(/1
e 4
1760
1780
1800
1820
Wavenumber (cm-’)
Figure 4. Observed IR-indudand thermal processesin the CO stretching region of fluoroacetic acid in K r (a) after deposition at 27 K, (b) after 20 h of IR irradiation; (c) after annealing at 42 K for 10 min; (d) after 20 h of IR irradiation. 4
I
I/
a I
.
n
1765
1775
A
1785
1795
Wave n u m be r
1805 (cm-’ )
1815
Figure 5. Observed IR-induced and thermal proccss in CO stretching region of fluoroacetic acid in Xe host: (a) after deposition at 37 K, (b) after 18 h of IR irradiation; (c) after annealing at 45 K for 10 min; (d) after 50 h of IR irradiation.
at 25 K was needed to complete the thermal process in Ar and Kr. In Xe the corresponding process was observed to start at 16 K. The bands can be divided to three classes based on the annealing behavior: those increasing during annealing (I) (for example the band at 1806 cm-I), bands decreasing during annealing (11) (for example the band at 1773 cm-1) and bands showing no change during annealing (111) (for example the band 3505 cm-l) in Ar. During IR excitation of a Kr matrix deposited at 18 K, bands I1 and I11 increase and the bands I decrease. If these samples were subsequently annealed at 22 K after IR excitation the
Nieminen et al.
10928 The Journal of Physical Chemistry, Vol. 97, No. 42, 1993
I
TABLE II: Thermal and IR-Induced Changes in Absorption Bands (cm-I) of Chloroacetic Acid in Ar, Kr, and Xe Matrices' Ar 3569 w 3566 w 3560 w 3555 vw
Kr 3552 m
Xe 3552 w
*
3546 vw
3545 vw 3538 vw
3540 vw
3505 vw
3496 vw
1806 s 1803 m
1803 s
1789 vw
1786 vw
1775 vw 1773 vw 1770vw
1767 w
1746 1742 1731 1408 vw
1428 vw 1412 vw 1366 vw 1362
1369 vw 1359
1354 w 1333 vw 1321 vw
1352 vw 1329 vw 1321 vw 1281 vw 1270vw
1 1 1 1 vs 1098 vw
1108 vs 1104vw
929 vw 891 vw 795 vw 792 m 790 vw
927 vw 888 vw 795 vw 793 w 790 vw
1407 1403 vw 1400 vw 1360 vw
I* I I1
1354 1349 vw 1325 vw
I* I I11 I I I11 I1
1317vw 1278 vw 1266 vw
1191 1184 1158vw 1129vw 1114 1111 1105 vs lloovw 944 929 vw 887 vw 797 vw 790 vw 792 773 vw 647 vw 625 618 414
770 vw 649 vw
611 m 609 m 605 m 584 vw
I* I1
1241 1194vw 1154vw 1135vw
613 vw 608 m 605 vw 596 vw
I1 I11
3535 3531 vw 3526 vw 3486 vw 3479 vw 1800 vs 1797 1782 vw 1778 vw 1769 vw 1765 w 1762 w 1760 1737 1728
1220
1157vw 1136vw
I1
*
3544 vw
1269 vw 1260 1220 1217 1194vw
I I
610 vw 604vw 583 vw 506 500 489 w
I11 I I' I11 I1
* * *
* * * I * *
I1 I1 I* I* I I
*
I I I I* I11 I1 I* I I1
I* 492 w 492 w I a I: decrease in IR excitation, increase in annealing. 11: decrease on annealing, increase in IR excitation. 111: change in IRcxcitationdepends on initial situation, no change in annealing. * associated form.
intensity of bands I increases, intensity of bands I1 decreases and the intensity of bands I11 did not change. For example in the C=4 stretching region, the intensity of bands at 1767 and 1786
1760
1780
1800
1820
Wavenumber (cm-' ) Figure 6. Observed IR-induced and thermal process in CO stretching region of chloroacctic acid in Kr: (a) after deposition at 18 K (b) after 40 h of IR irradiation; (c) after annealing at 22 K for 10 min. cm-1 increase and the bands at 1803 cm-I decrease during IR excitation. During annealing the bands at 1767 cm-I decrease, bands at 1803 cm-1 increase, and the band at 1786 cm-' did not change. Figure 6 displays this process monitored in the C - 0 stretching region. During IR excitation of a Kr sample after the IR excitation and annealing treatment described above, a very peculiar process was observed. The intensity of bands I decreases and intensity of bands I1increases as expected. However, the intensity of bands I11 first decreases rapidly but after reaching a minimum it started to increase slowly. Figure 7 displays this process for three bands. A similar process was observed also in Ar matrices after deposition at 20 K. From these experimentsit was concluded that direction of change of species I11 during IR excitation correlates with the concentration ratio II/III. In Xe new bands (labeled with I*) appear in the spectrum when deposition was carried out at 37 K. If the deposition was carried out at a lower or higher temperatures the intensity of these bands decrease dramatically. Figure 8 displays the IR spectrum after deposition of sample at 22 K, after deposition at 37 K, after annealing the sample deposited at 37 K for 10 min at 65 K, and after deposition at 47 K. These new bands behave like type I bands during IR irradiation, and annealing of the sample at 65 K begins a process in which these bands disappear and the intensity of bands I increases. These bands (I*) are ca. 2-7 cm-1 shifted from the bands attributed to species I, except those assigned to v(0H) at 3535 cm-1 (shift 17 cm-I) and T(OH) at 618 cm-I (shift ca. 8-15 cm-1).
III. Ab Initio Calculations Computational Details. The calculations were carried out by using the Gaussian 90" program on Convex C3840 and Cray X-MP EA1432 computers. Electroncorrelation and polarization
The Journal of Physical Chemistry, Vol. 97, No. 42, I993
Fluoroacetic and Chloroacetic Acid
w
1349
1350
1351
10929
I 1352
1353
1354
(d
e
0 v)
P 4 O 0.075 7
Wavenumber (cm-' )
Figure 7. Observed of IR-induced processes in the typical bands of chloroaceticacid. Solid line: after subsquential40 h of IR irradiation and annealing at 22 K for 10 min of sample deposited at 18 K. Dashed line: after 10 h of IR irradiation (from situation of solid line). Dotted line: after 58 h of IR irradiation (from situation of dashed line).
should be taken into account when calculating energies and geometries of compounds possessing intermolecular interacti0ns,~~-15 such as FAA and CAA. The basis sets used for optimizations were the standard split valence bases including polarization: 4-31G*, 6-31G**, 6-311G**. The electron correlation was taken into account via the second-order MerllerPlesset perturbation theory, and the correlation included all electrons. The torsional potential surface was calculated at the MP2/6-31G** level. The harmonic frequencies for each conformer were calculated numerically at the MP2/4-3 lG* level of theory, and the corresponding Cartesian force field was used as an input for normal-coordinate analysis. Theoptimizationat theMP2f6-31 lG**leveloftheoryyielded four conformers for both CAA and FAA (see Figure 9). The following nomenclature is used in labeling the calculated conformers: an upper-case letter designates the dihedral angle 0-C-C-X (X = F, C1; C, cis; T , trans), Le., the internal rotation around the C-C bond, and a lower-case letter designates the dihedral angle H-O-C-C, Le., the OH torsion. (Gt is an approximate description; 0-C-C-X = 77.1'). The energy barriers between conformers are quantitis of interest. As previously noted,' the pictures of potential energies versus one torsional coordinate are insufficient to describe all possible paths for conformational interconversions. Figures 10 and 11 display the MP2/6-31G** level simulated (mainly rigid) torsional potential energy surfaces as a topographic map for both acids. For both acids, more than 150 points were calculated to create these surfaces. The minima and maxima were fully optimized. Although these surfaces give only a rough qualitative picture of the real situation, they provide a very useful way to find out the lowest energy channel for conformational interconversion. For most stable conformers the barriers hindering internal rotation of the substituted methyl group around the C-C bond
3450
3500
3550
3600
Wavenumber ( c m - ' )
Figure 8. Spectrum of matrix isolated chloroaceticacid in Xe: (a) after deposition at 22 K (b) after deposition at 37 K (d) after annealing of sample; (d) at 65 for 10 min; (c) after deposition at 47 K.
were calculated with full geometry optimization at every sampling point. Other torsional potentials were calculated by using the values of the nearest optimized structure for other coordinates. However, all barriers between obtained conformers were fully optimized. The energies of optimized conformers and barriers (relative energy and dihedral angles 0-C-C-X, H-O-C-C) for conformational interconversion are given in Table 111. Fluoroacetic Acid. The energetic order of four conformers of FAA was consistent at all levels of theory used (see Table I). All conformers have C,symmetry. In agreement with the previous ab initio studies,6v7the conformer Tt with values 180' for both dihedral angles H-O-C-C and F-C-C-O was found to be the most stable form for FAA. The second lowest energy species, Cr, differs from Tt by 180' internal rotation of the C-C bond. The MP2/6-311G** calculated energy difference between Tt and Cr is ca. 1 kJ mol-' and the MP2/6-31G** calculated barrier for Tr Ct interconversion is ca. 13 kJ mol-' (see Figure 12 and Table 111). The barrier of ca. 56 kJ mol-' for internal rotation of the OH group about the 0-C bond leading from Tr to Tc is higher than the barrier for rotation of methyl group about the C-C bond (see Table I11 and Figure 13). The conformer Tc is the highest energy conformer for FAA and the energy difference ca.3 1 kJ mol-' between Tr and Tc is too high for Tc to be observed experimentally under present conditions. The H0-C internal rotation barrier of Ct is also high. However, the resulting form Cc is only ca. 4 kJ mol-' higher in energy than Tr. TheoptimizedgeometriesofbothacidsattheMP2/6-311G** level are given in Table IV with the previously reported gas phase data.2.4 For FAA the calculatd structures of the conformers Tt and Cc are in excellent agreement with the previous gas-phase data.2 The comparison shows that all values except the 0-H bond length for Cc differ less than 1%. However, as previously noted,' the reported gas-phase value 93.8 ppm for the 0-H bond
-
10930 The Journal of Physical Chemistry, Vol. 97, No. 42, 1993
X
Nieminen et al.
X
2
-90
-45
0
45
90
135
180
225
270
D i h e d r a l a n g l e HOCC Figure 11. MP2/6-31G** calculated torsional potential energy surface of chloroacetic acid along dihedral angles F-C-C-O and H-O-C-C. Contours are drawn for 8 kJ mol-' starting from 5 kJ mol-'.
Gt FIgm 9. MP2/6-311G** calculated conformers of fluoroacetic acid and chloroacetic acid. Both acids have the common conformers Tz, Cc, and Tc. In addition fluoroacetic acid has a minimum corresponding form CZand chloroaceticacid has a minimum corresponding to structure Gt. X designates a halogen atom.
some differences in the conformer relative energies. These are most probably due to the fact that conformers were not optimized completelyand the computationallevel was lower in the previous study. chloroacetic Acid. For CAA two trans (HOCC = 180°) and two cis (HOCC = Oo) conformers were found. In agreement with previous molecular mechanics8 and a6 initio' studies the conformer Tt with Cssymmetry was found to be the most stable form for CAA. For CAA, as for FAA, the second lowest energy species Gt differs from Tt by internal rotation of methyl group about the C-C bond. However, as can be seen from Figure 14 the dihedral angles of minima and maxima are not the same as for FAA. Instead of finding conformer Ct there is maximum in torsional potential at 8 = 180° and the minima in addition to Tt are at 8 = 64' and at 8 = 296O, namely, conformers Gt and Gt'. However, it should be noted that both Gt and Gt'conformers are spectroscopically identical and they behave similarly on the observed processes. Although the 8 value for Gt changes to 7 7 O (or to 283O) when optimization was done at the MP2/6-311G** level, no new conformers where found. The calculated barrier of ca. 5 kJ mol-1 for internal rotation around the C-C bond (processes Tt Gt and Gt Gt') is much lower than the barrier (13 kJ mol-') for the corresponding process Tt Ct in the case of FAA. As for FAA the calculations suggest that there are two conformers with the dihedral angle HOCC = Oo (0 = OO): the species Tc and Cc. Figure 15 displays the MP2/6-31G** calculated conformational energy for CAA as a function of the dihedral angle HOCC. The MP2/6-3 1G** calculated relative energy of 30 kJ mol-' for Tc and the barrier of 56 kJ mol-' for the process Tt Tc are somewhat higher than previous molecular mechanics values*of 25 and 42 kJ mol-', respectively. However, the relativeenergy of Tc is probably too high for Tc to be observed experimentally. For Cc the relative energy of 14 kJ mol-' and the barrier of 51 kJ mol-' for process Gt Cc were obtained. The MP2/6-311G** calculated structural parameters of Tt are in good agreement with gas-phase data.4 For other conformers there are no gas-phase data available.
-
-90
-45
0
45
90
135
180
225
270
D i h e d r a l a n g l e HOCC Figure 10. MP2/6-31G** calculated potential energy surface of fluoroacetic acid along dihedral angles F-C-C-O and H-O-C-C. Contours arc drawn for 9 kJ mol-' starting from 4 kJ mol-'.
length is rather short for FAA. In all, considering the good reliability of the ualculated structures of species Tt and Cc, it seems to be reasonable to assume that calculated values for Tc and Ct are also reliable. Finally, it is interesting to note that our calculated value for the C-C bond length supports the conclusion of Van Eijck2 that there could be some error in the previous gas-phase result.1 Our calculations for FAA are in qualitative agreement with previous double-f SCF calculations: i.e., the energy order the of the conformers is the same. However, comparison of the present results with those obtained at the doubler SCF level indicates
-
-
-
-
IV. Discussion Fh"cetic Acid. IR-induced and thermal conformational processes in matrix-isolated FAA were found in all hosts. On the basis of experimental data, Bames et a1.16 correlated the barrier
The Journal of Physical Chemistry, Vol. 97, No. 42, 1993 10931
Fluoroacetic and Chloroacetic Acid
TABLE ILI: Calculated Energies of Conformers of Fluoroacetic Acid and Chloroacetic Acid at Various Levels' Fluoroacetic Acid level of conformer calculation Tt ct cc ~
6-31G** MP2/4-3 1G* MP2/6-31G** MP4(STDQ)/6-3 1G** //MP2/6-31G** MP2/6-31G**
Tt
MP2/6-31G**
-
-326.659 501 -321.133 661 -321.461 922
0.32 0.81 0.19
1.46 4.25 4.01
Tc 36.24 34.51 32.88
-321.499 088 -321.109 97
1.01 0.99
3.62 5.53
31.30
Ct
Tt
12.18 (100, 180)
-
Barriers Tc
ct
56.61 (180,81)
-
cc
Cc-
58.69 (0,90)
Tc
44.65 (111,O)
Chloroacetic Acid level of calculation
Tt
Gt
conformer
6-31G** MP2/4-3 1G* MP2/6-31G** MP2/6-3 lG**
-686.192 188 -686.198 509 -681.491 204 -687.726 816 3
1.28 2.11 2.33 1.25
+.
Tt Gt 5.16 (116, 180)
MP2/6-3 1G**
Tt Tc 56.35 (181,85)
cc 11.18 13.64 15.65 13.93
-
Barriers
Gt Gt 4.10 (0,180)
Tc 34.13 32.99 31.53 29.98
-
Gt Cc 52.10 (268,58)
Cc- Tc 38.55 (121, 3)
a The absolute energy is given for the lowest energy conformer (in au), the other energies are the relative energies (in kJ mol-') referring to the energy minimum. The numbers in bracket are coordinates X-C-C-O and H-O-C-C,respectively.
0
60
120
180
240
300
360
Dihedral Angle F-C-C-0 Figure 12. MP2/6-31G** calculated conformational energy for FAA as a function of the dihedral angle F-C-C-O. cp designates the dihedral angle H-O-C-C.During calculation of torsion Tt Ct only dihedral angle F-C-C-O was frozen other coordinates was fully optimized. The torsion Cc Tc was calculated as rigid torsion. Optimized conformers and maxima (a: Tt Ct; 8: Cc Tc) are marked with 0, 0 .
- -
-
-
to internal rotation and the temperature needed to stabilize the species in different hosts. This relation yields for observed thermal processes the barrier height between conformers I and I1 to be ca. 12 kJ mol-1. This value is essentially the same as that calculated for the Ct Tt interconversion, Le., the C-C torsion of the fluoromethyl group. Furthermore, the MP2/6-311G** calculatedvalue of ca. 1 kJ mol-' for the energy differencebetween Tr and Ct is in excellent agreement with our matrix value for observed conformers I and 11. Therefore, from the energetic point of view the observed bands I can be confidently assigned to conformer Tt, and the observed bands I1 can be assigned to conformer Ct. Van Eijck et al. have studied the rotamers of FAA by electron diffracton and microwave techniques.'J They found only two conformers Tt and Cc in the gas phase. Although Ct species was not found in the previous gas-phase microwave studies,l.2 our result does not condradict those studies. As Van Eijckl pointed out, due to the small dipole moment of Ct compared to that of Tt, the conformer Ct would probably be undetected by the microwave technique even if C t were more abundant.
-
Dihedral Angle C-C-O-H Figure 13. MP2/6-31G** calculated conformational energy for FAA
as a function of the dihedral angle H-O-C-C. 0 designates dihedral angle F-C-C-O. The torsions were calculated as rigid torsions. Optimized conformers and maxima (a:Tt Ct; 4: Cc Tc; u: Tr -c Tc; y: Ct Cc) are marked with 0, 0 ) .
-
-
-
The wavenumbers of the observed bands at 3566 and at 3555 cm-1 are typical for "free" OH group in carboxylic acids. For example, the OH stretching frequency of the corresponding conformer of glyoxylic acid17 is 3555 cm-l, and in acetic acidlo it is 3566 cm-1 in Ar. The OH stretching band of conformer Cc with internal hydrogen bond (H-F) would probably be at lower frequency. For CAA, the corresponding band was found at 3505 cm-I. For FAA no OH stretching absorption of Cc conformer was found. However, the weak v(C0) band at 1804 cm-1 could be due to this form for FAA. The very slow IR-induced process and the absence of thermal processes(monitored by the 1804-cm-1 band) could be due to the high barrier for conformational interconversion. Finally, comparison of v ( C 4 ) and v(0H) absorptions of FAA and acetic acid10 in Ar shows very interesting similarities. The wavenumber of the v(0H) band at 3566 cm-I, assigned to Tt, and of the v ( C 4 ) band at 1779 cm-I assigned to Cr, are the same as for acetic acid. This observation is in agreement with the structure of the conformers (see Figure 11). For conformer Tr fluorine is in trans position to the hydroxyl group
Nieminen et al.
10932 The Journal of Physical Chemistry. Vol. 97, No. 42, I993
TABLE Iv: MP2/631G**Optimized Geometries of Fluoroacetic Acid and-ChioroaceticAcid' coordinate rrons(Tl)b c i s ( C ~ ) ~ Tt Ct Cc Hi02 98.3 h 1.2 93.8 h 0 . 6 96.6 96.7 96.6 136.0h0.5 120.1h0.5 C3C4 149.7 h 0.4 C4Fs 136.5h0.6 C4H7,8 109.7 h 0.3 HlO2C3 105.6 i 0.8 C4c306 126.9 h 0.4 02C3c4 109.5 h0.5 C3C4Fs 110.2h0.4 C ~ C ~ H T109.6h ,~ 0.3
135.3 120.1 151.4 136.8 109.3 105.7 126.2 109.1 110.3 108.9 FF4H7,s 109.8 H T C ~ H ~109.0 h 0.3 110.0 h 0.3 109.2 180.0 0 H102CnC4 180 180.0 0 OzC3C4Fs 180 180.0 180 06C302c4 180 c306
coordinate Hi02 Ozc3 COO6 c3c4
C4Ch C4H7 C4HS H102C3 C4c306 02C3C4 C3C4Clj CGHi CpC4Hs
133.7h0.6 120.7h0.5 151.0 h 0.5 139.0h 0.4 109.4h 0.3 108.4 h 0.3 120.8 h 0.6 116.6h 0.4 110.9h0.4 109.3 h 0.3
134.1 120.7 151.5 137.2 109.2 105.1 121.9 113.2 113.2 107.6 109.6 109.2 180.0 0.0 180.0
134.2 120.1 152.3 139.1 109.0 107.0 121.1 115.2 111.2 108.8 109.0 110.0 0.0 0.0 180.0
Tc 96.2 136.0 119.6 152.3 135.5 109.5 109.7 124.7 113.5 110.6 109.1 109.4 109.2 0.0 180.0 180.0
rrtans (Try
Tr
Gt
cc
Tc
97.0 (1.5) 135.2(0.5) 122.3 (0.4) 150.8 (0.6) 177.8 (0.5) 109.0 (0.2) 109.0 (0.2) 105.8 (1.1) 126.1 (0.5) 110.6(0.4) 112.5(0.4) 109Sd 109.5'
96.6 135.4 120.1 151.6 176.0 109.0 109.0 105.5 127.6 108.2 112.8 108.4 108.4 109.4 109.4 108.5 180.0 180.0 180.0
96.7 134.7 120.6 151.5 177.9 108.7 108.9 105.7 124.7 111.0 109.8 108.6 110.4 108.4 108.4 111.8 182.4 77.1 180.1
96.6 133.9 120.2 153.0 178.0 108.9 108.9 107.7 119.1 117.9 115.3 107.7 107.7 108.4 108.4 109.2 0.0 0.0 180.0
96.2 136.0 119.5 152.7 175.8 109.2 109.2 109.7 126.1 112.6 113.1 108.8 108.7 108.8 108.8 108.6 0.0 180.0 180.0
C1dU-b
ClsGHs H7GHs 109Sd H102CoC4 180" 180, 101,48 02C3C4Cls 180" 06C302c4 0 The bond lengths are in picometers, angles are in degrces, and the atom numbering is given in Figure 9. b Gas-phase results from ref 2. Gas-phase results from ref 4. d Assumed value.
=
50
1
1
1 40t
Tt 0
60
120
180
240
300
360
Dihedral Angle CI-C-C-0 Figure 14. MP2/6-31G8* calculated conformational energy for CAA as a function of the dihedral angle Cl-C-C-0. cp designatesthe dihedral angle H-O-C-C. During calculation of torsion Tt Gr Gr only dihedral angle Cl-C-C-0 was frozen other coordinates was fully optimized. The torsion Cc Tc was calculated as rigid torsion. Optimized conformers and maxima (a: Tr Gr; a': Gr Gt'; 8: Cc Tc) are marked with 0,0.
-
-
- - -
and the intermolecular interaction is small (perturbation to the v(OH) is small). For conformer Ct,fluorine is also in trans position to carbonyl oxygen and the perturbation to the v ( C I 0 ) is small. Chloroacetic Acid. In agreement with the previous matrix study5 our data indicates the presence of three conformers for CAA in the matrices. IR-induced processes between all three conformers and a thermal process between species I and 11were observed (the threshold for thermal processes being ca. 18 K in
i
e,
01 0
60
120
180
240
360
300
Dihedral Angle C-C-0-H Figure 15. MP2/6-31G** calculatedconformationalenergyasa function of the dihedral angle H-OC-C. B designates the dihedral angle C1C-C-O. The torsions were calculated as rigid torsions. Optimized conformers and maxima (8: Cc Tc; u: Tt Tc; y: Gt Cc) are marked with 0,0 . Ar and Kr). By using the Barnes relation,16 the barrier for the observed thermal process between forms I and I1 is essentially the same as calculated for process Gt Tt (ca. 6 kJ mol-'). Furthermore, the existence of such a low barrier can only be attributed to the rotation of the substituted methyl group. Therefore, the observed bands I and I1 can be assigned to species Tt and Gt, respectively. However, the bands of type I11 were essentially insensitive to annealing up to temperatures near 70 K (in Xe). In agreement with previous assignments these bands were assigned to conformer Cc with possible intramolecular hydrogen bonding. The assignment of Cc is in agreement with the fact that the observed v(0H) frequency is a ca. 50 cm-l lower wavenumber than that of Tt and Gt, indicating weak H-C1 interaction. Similar v(0H) shifts have been found for several conformers of chloroethanols.l* Irradiation CAA in Ar with a CO laser line at 1805 cm-l was found to cause a process where both Tt and Gt decrease and Cc increase.5 This and the annealing experimentsgive an estimation that the barrier for conformational interconversion from Gt to Cc is near 22 kJ mol-'. Thisvalue is somewhat lower than our MP2/ 6-31G**calculated value 5 1 kJ mol-'. However it is known that the effective barrier for internal rotation of the OH group can be lower than calculated due to tunneling. On the other hand, by using the calculated intensities for v(C=O) (for all conformers nearly the same) the relative energies of ca. 1.4 and 8 kJ mol-' was obtained for Gt and Cc, respectively. These values are in satisfactory agreementwith our MP2/6-3 11G**calculatedvalues of ca. 1.2 and 14 kJ mol-'. Previous gas-phase data4 suggests presence of three trans conformers (HOCC = 180') with values of 180°, l o l o , and 48O for dihedral angle Cl-CC-0. This result is in disagreement with our calculations, as well as with present and previous5 matrix results for CAA, having only two trans conformers. However the potential energy function for rotation of the methyl group is rather
