J. Phys. Chem. 1993,97,8608-8616
8608
Urea Protonation: Raman and Theoretical Study Nanping Wen and Murray H. Brooker' Chemistry Department, Memorial University of Newfoundland, St. John's, Newfoundland, Canada AI B 3x7 Received: March 25, 1993
Urea is known to protonate in aqueous acid, and it will even form solid compounds with strong acids, e.g., OC(NH2)2*HCl. Raman spectroscopic studies have been performed on aqueous solutions of urea and urea-lSN2 in hydrochloric acid. The changes in peak frequency that accompany urea protonation indicated that the point of proton attachment is carbonyl oxygen, which is consistent with previous N M R and neutron diffraction studies of uronium salts. The equilibrium constant for the protonation of aqueous urea has been measured by quantitative Raman intensity measurements of bands due to urea and protonated urea, K = 1.93 L-mol:l. Theoretical ab initio calculations of minimum energies and optimum geometries have been performed for urea, for protonated urea (N and 0 isomers), and for protonated urea (0 isomer) hydrated to one water molecule. The ab initio calculations were in excellent agreement with experiment.
1. Introduction
Urea and other amides are important biological molecules because of potential hydrogen-bonding and acid-base properties associated with both the amine and carbonyl groups. A large number of spectroscopicl-11and thermodynamicl2-20 studies have been performed in an attempt to assimilate information about the physicochemical properties of aqueous solutions of urea and other amides. Free urea is a planar molecule of Cz, symmetry. Crystal structure studies21922 have shown that in solid urea both nitrogen atoms are identical. Bond-length measurementsin urea gave the C-N distance as 1.37 A, which is shorter than a typical single bond and has been interpreted to indicate that the C-N bond has some double bond character (about 28%).23 Although urea appears to be a simple molecule, the compound has versatile characteristics. Urea is known to protonate in aqueous acid and even form solid compounds with strong acids, e.g., H2NCONH2.HCl. The observed basicity led to the early conclusion that proton transfer for aqueous solutions of urea with strong monobasic acids occurred at the nitrogen of the amide group not at the carbonyl oxygen. Previous infrared and Raman studies of urea nitrate192 were interpreted in terms of monoprotonation at N, and a theoretical justification was also advanceda2However, NMR studies by Redpath and Smiths for a number of urea salts were consistent with protonation at the carbonyl oxygen. The structure of uronium nitrate (urea nitrate) has been determined by neutron diffraction studies by Worsham and Busing.24 The acidic proton was found on the carbonyl oxygen and hydrogen bonded to nitrate. Raman spectroscopy has not been widely used for quantitative equilibrium studies because relatively high concentrations are required for suitable intensities, but for equilibrium constants near unity and for fast exchange equilibria Raman can be the method of choice. In the present study Raman spectroscopic measurements of aqueous solutions of urea and ureaJSN2 in hydrochloric acid have been performed. The results revealed that in these solutions urea combines with H+ ion through its carbonyl oxygen and not through the nitrogen of the amine group. The thermodynamic equilibrium constant of urea protonation has been measured for the first time. Since amide and carbonyl groups are prevalent in biological molecules and often undergo interactions with H+ion, urea can serve as a good simple model molecule for measurements and for theoretical calculations. 2. Experimental Section
The spectroscopic measurementswere carried out on a Coderg PHO Raman spectrometer after sample excitation with an
INNOVA 70-4 argon ion laser operating at approximately 300 mW output at the sample for the 488.0-nm wavelength line. The usual right angle scattering geometry was employed with the incidentlight vertically polarized and the scattered light analyzed with polaroid film to give the41[x(zz)y]andZl [x(zx)y]scattering geometries. A quarter-wave plate before the entrance slit served to compensate for the grating polarization preference. Raman wavenumber calibration was achieved with the plasma lines of the laser, but when required a narrow-bandpassinterferencefilter was used to remove the unwanted plasma lines. The spectral slit widths of the double monochromator were set at 2.0 cm-l, which is sufficiently narrow to avoid the need for a slit correction to the bandshape. The Raman scattering light was detected with a PMT cooled to -30 OC, integrated with a photon counter, and processed with a homebuilt boxcar averager interfaced to the Memorial University VAX 8800 computer. The scan rate was 50 cm-1 min-I, and two data points were collected per wavenumber. Spectra were signal-averagedand smoothed twice with a SavitskyGolay three-point smoothing function. A program was applied which corrected the measured intensity for the fourth power frequency factor and then set the lowest data point to zero and the highest data point to 999 on a relative intensity scale. This form of the data is defined as our Z(o) spectrum which should be independent of excitation frequency. The same program was applied with the option to correct for the fourth power scattering factor, the Bose-Einstein temperature factor, B = [ 1-exp (-hcw/ k n ] ,and the frequency factor, o,to give the reduced or RQ(o) spectrum which is directly proportionalto a point by point relative scattering activity, SQ(O),in terms of mass weighted normal coordinates, Q,in the double harmonic approximation. RQ(u) is the form of the Raman spectrum that most closely approaches the vibrational density of states.25.26 The relationship between the Z(w) and RQ(u) forms of the spectra is given by eq 1. For
quantitative studies it is preferable to plot the spectrum in the RQ(w)form because the Bose-Einstein factor removes the statedependent temperature factor of the excited-statetransitions and leaves the effect that is due to concentration changes. Only in the low-frequency region are the Z(w) and RQ(u) spectra significantly different. Curve resolution of overlapped bands and calculations of band half-width and intensity were achieved with a standard computer fitting program. All chemicals were reagent grade and used without further purification. The samples of urea (A.C.S. reagent) and urea15N2 (99.8 atom % l 5 N ) were obtained from Fisher Scientific
0022-3654/93/2097-8608$04.00/0 0 1993 American Chemical Society
T h e Journal of Physical Chemistry, Vol. 97, No. 33, 1993
Urea Protonation
8609
TABLE I: Observed Raman Frequencies and Assignments of 2.00 mkL-l Urea and Urea-lSN2Aaueous Solutions ~~
urea
urea-I5Nl
urea.H3@
1662 1597 1464 1158 1003 58 1 522
1656 1589 1461 1148 982 578 517
1640 1560 1497 1120 1015 575 519
~rea.H3@-'~N2 1635 1568 1493 995 567 510
* For neutral urea the assignment is based on the following reference: Stewart, J. E. J. Chem. Phys. 1957, 26, 248. For protonated urea the assignment is referred to comparable modes in urea.
cv) Figure 1. Possible structures (canonical forms) of urea.
Company and MSD Isotopes, respectively. The hydrochloric acid (analytical reagent) was made by Mallinckrodt Inc.. All aqueous samples were prepared with distilled water and sealed off in quartz tubes (4 mm in outside diameter and approximately 15cm in length). A 5-min centrifugation was performed to settle dust particles before the spectrum was recorded. All spectra were recorded at room temperature. The room temperature was maintained at 22 f 1 OC by building maintenance service. No additional device used for sample temperature control. There was no evidence to suggest that the sample was warmed by the laser. The spectra were not affected by changes to the laser power. Theoretical calculationswith ab initio methods were performed at the Hartree-Focklevel on MUNGAUSS27 to obtain minimum energies, optimized geometries, and electron densities.
3. Results and Discussion Molecular Structure. Urea might conceivably exist in an enol form (I in Figure l),as a strict carbonyl compound (11), or as a resonance hybrid of structures 11,111,and IV2*with eachcanonical
form contributing roughly an equal amount as a superposition of structures 11, 111, and IV, i.e., structure V. X-ray diffraction studies of urea2lSz2showed the C-N bonds to be equivalent, which rules out structure I, as does the observation of four N-H stretching vibrationsin the infrared spectrum of ~ r e a , * ~ Kumler ,~O and Fohlen3*reported dipole moment measurements that were consistent with a 20-30% charge character separation (structures I11 and IV). The X-ray studies21922 yielded bond distances of 1.37 A for the C-N bonds and 1.25 A for the C-0 bond in urea. These are seen to be intermediate between the normal single and double bond distances: C-N, 1.47 A; C-N, 1.26 A; C-0, 1.43 A; and C=O, 1.22 A. This adds evidence in support of the superposition structure (V). The superposition hybrid structure (V) is consistent with a planar configuration because the central carbon atom probably has a hybrid trigonal sp,py electronic configuration with a pz orbital perpendicular to the N2CO plane. This pzorbital overlaps the oxygen pz orbital forming a ?r bond. Now the nitrogen electronic structure also involves a trigonal sp,py hybrid with a pair of pz orbitals perpendicular to the HNH plane. If the molecule is planar these nitrogen pz orbitals can also overlap the carbon orbital, giving a partial double bond character to C-N links and reducing the bond order of the C-O links. The planarity of urea has been verified by the proton magnetic resonance studies of Andrew and H ~ n d m a n In . ~the ~ following discussion urea will be considered as planar molecule of CZ, symmetry. Vibrational Asssignment. A free urea molecule, with planar Czv structure, has its fundamental vibrations distributed in
1000.0 900.0 800.0
700. 0-
> +
b
800.0-
H
*
z w +z H
500.0-
400.0-
300.0200. 0 -
100.0-
I
0.0 0.0
I
I
I
I
I
I
I
250.0
500.0
750.0
1000.0
9.0 1250.0
1500.0
1750.0
WAVENUMBER
Figure 2. Raman spectra of 2.00 mol-L-I urea plotted as (a) I(w)ll and (b) I(o)*, at 295 K.
1 2000.0
Wen and Brooker
8610 The Journal of Physical Chemistry, Vol. 97, No. 33, I993 1000.0
800.0
I
700.0
>
600.0
c
-la I-
z
H 300.0 200.0 100.0 0
i \\
. 0. 0
II
0 4 250.0
4 500.0
750.0
1000.0
1250.0
WAVENUMBER Figure 3. Raman spectra of 2.00 mo1.L-Lurea in 37% HC1 plotted as (a) I(w)li and (b)
1500.0
1750.0
2000.0
at 295 K.
1000.0
1
900.0800.0n
3
700.0-
W
600.0-
> I-
500.0-
H ul
z
W
400.0-
300 0 200.0100.0-
0.0
0.0
100.0
200.0
300.0
400.0
500.0
600.0
700.0
800.0
WAVENUMBER Figure 4. Raman spectra of 2.00 mo1.L-I urea in 37% HCl plotted as (a) Rp(w)ll, (b) Re(@)*,and (c) R Q ( u )-~ R ~ Q ( u ) ~at, 295 K.
+
symmetry species as follows: 7A1 + 2A2 3B1 + 6Bz. All vibrations are Raman active, but the A2 vibrations, a NHz wagging, and a NH2 torsion oscillation are inactive in the infrared spectrum. The polarized and depolarized Raman spectra in the region between 80 and 2000 cm-1 of aqueous solutions of 2.00 mo1.L-1 urea at room temperature are shown (Figure 2). The Raman spectra for an aqueous solution of 2.00 mo1.L-LureaJ5N~ at room temperature were similar but showed the expected frequency shifts. The observed Raman frequencies and corresponding assignments for 2.00 mol-L-' urea and urea-'5Nz have been collected in Table I. Stewart33assigned the 1665-cm-1 band to a CO stretching vibrationandthe 1593-cm-1bandtothein-phaseNH2deformation mode. The symmetric A1 and antisymmetric BZN-C-N bond stretching vibrations were placed at 1003 and 1464 cm-1, respectively, the former corresponding to a very strong, polarized
Raman band and the latter to a weak and broad depolarized Raman band. The assignment of these frequencies to the N-C-N stretching vibrations of A1 type and B2 type was confirmed by a calculation of the potential energy di~tribution.3~ StewarP assigned the 1153-cm-1 band to the NH2 rocking vibration of AI type and a broad band centered at 1064 cm-1 to the same type of vibration of Bz type. However, Yamaguchi, Miyazawa, Shimanouchi,and Mi~ushima3~ assigned the rocking vibration of both A1 and B2 types to the band at 1150 cm-1, on the basis of a normal coordinate vibrational calculation. The Raman band centered at 1064 cm-1 was not observed in this study. The antisymmetricaldeformation of N-C-N was placed at about 581 cm-I. The Raman band at about 525 cm-1 was attributed to the B1type NHI torsion oscillation. Urea Protonation. Urea possesses two basic sites which might conceivably accept protons: an ammonium ion (proton addition
The Journal of Physical Chemistry, Vol. 97, No. 33, 1993 8611
Urea Protonation 1OOO. 0
900.0
800.0 700.0
>
Boo.0
H
A
v)
z
500.0
w
fi
I\
A
t-
g
400.0
e
900.0.
d C
200.0,
b
100. 0 .
0. 0 . OM.O
875.0
900.0
925.0
SSO.0
975.0 1000.O 1025.0 1050.0 1075.0 1100.0
WAVENUMBER Figure 5. Raman spectra of 1.OOO mo1.L-Lurea with (a) 0.100 mol-L-I HCl, (b) 0.500 mol-L-l HCl, (c) 1.00 mo1.L-I HCl, (d) 1.50 mo1.L-l HCl, (e) 2.00 mo1.L-I HC1, (f) 3.00 mo1.L-l HCl, and (8) 6.00 mol-L-I HCl, at 295 K. 1OOO. 0 -
A
I dl
900.0-
800.0-
700.0-
t
600.0-
L
"1
kJ
=-*
H
4oo.oi
2
II
u
0. 0 850.0
875.0
900.0
925.0
950.0
111 Ill
875.0
1OOO. 0 1025.0 1050.0 1075.0 1100.0
WAVENUMBER Figure 6. Raman spectra of (a) 0.5000 mo1.L-1, (b) 0.7500 mol-L-', (c) 1.OOO moleL-', (d) 1.250 mol-L-l, (e) 1.750 mol-L-l, and (f) 2.000 mo1.L-l urea with 0.500 mol-L-I NaClO4 plotted as Re(@)!,at 295 K.
to the nitrogen atom) and an oxonium ion (proton addition to the oxygen atom). The polarized and depolarized Raman spectra between 80 and 2000 cm-l of an aqueous solution of 2.00 mo1.L-l urea in 37% HCl at room temperature is shown (Figure 3). The spectra of ureaJSN2 in 37% HCl exhibited the same major bands shifted to lower frequencies. Peak frequencies and assignments for protonated urea are collected in Table I. A strong Raman band at 1015 cm-1 in the spectrum of protonated urea (Figure 3) appears to be derived from the Ramanactive symmetricalN-C-N stretching vibration, VCN (AI). This new band due to protonated urea had a 12-cm-l shift to higher frequency compared to neutral urea (Figure 2). For protonated urea-lsN2, the in-phase 15N-C-IsN symmetrical stretching frequency (995 cm-1, Table I) was 13 cm-I higher than that in
neutral urea-ISN2itself (982 cm-I, Table I). The antisymmetrical B2 type N-C-N bond stretching vibration shifted from 1464and 1461 cm-I to 1497 and 1493 cm-l for protonated urea and protonated urea-ISN2, respectively (Table I). Thus one could interpret the shift of the N-C-N stretching to higher frequencies to indicate that the CN bonds have, if anything, more double bond character in the protonated urea than in the neutral urea. The C=O bond stretching vibration was placed at about 1640 cm-1 for protonated urea and at about 1635 cm-1 for protonated ureaJSN2 (Table I). Compared to neutral urea and ureaJSN2, the C=O stretching vibration shifted -26-27 cm-1 to lower frequency. It is obvious that the C 4 bond has less double bond character in the protonated urea than in the neutral urea.
8612 The Journal of Physical Chemistry, Vol. 97, No. 33, 1993
Wen and Brooker
TABLE Ik Ab Initio Calculations of Free and Protonated Urea
bond length (A)
bond order
3-21G
6-31G**
1.2 19967 1.364754 0.993738 0.995209 1.75081 0.90945 0.84991 0.83808
1.201795 1.359967 0.990124 0.991236 1.80613 1.02207 0.89500 0.887 16
3-21G
angle
122.7943 123.7110 117.4520
122.4277 123.6840 117.0463
atomic charge
-0.67267 1.15242 -0.94856 0.34204 0.36664 -222.736821
-0.475 0.93622 -0.77802 0.30148 0.32580 -223.998605
angle
114.5769 123.4419 118.6581 122.2392 121.8262 122.2063 120.5272
115.2793 122.4946 118.9061 121.9434 121.3406 121.7735 116.5844
atomic charge
-0.68862 1.34302 -0.91827 0.42217 0.44502 -0.93 171 0.42803 0.42253 0.4 7784 -223.097906
-0.53728 1.03083 -0.69838 0.37435 0.39278 -0.72410 0.37827 0.37425 0.40927 -224.358136
134.0006 114.8539 125.6091 117.5143 116.1245 107.3780 107.3780 109.2517 109.2517 -0.50184 1.09957 -0,92291 -0.90127 0.41054 0.44143 0.44128 0.46659 0.46659 -223.062906
132.1902 114.8165 125.1448 116.8781 117.3250 107.5073 107.5071 108.7697 108.7698 -0.44768 0.88409 -0.72381 -0.69386 0.35917 0.39178 0.39472 0.41829 0.41829 -224.321650
116.6593 123.55 16 118.2392 122.3476 122.2 106 121.1023
116.2751 122.6255 118.6241 121.8119 121.7 160 120.6996
energy (hartree)
bond length (A)
bond order
1.313808 1.304856 1.000137 1.003206 1.3 14003 1.00173 1 1.001221 0.968973 1.05297 1.15921 0.80126 0.78076 1.11746 0.79463 0.79955 0.73103
1.290597 1.305834 0.995513 0.995812 1.313115 0.996630 0.996235 0.9485 10 1.15182 1.25838 0.85617 0.84296 1.22003 0.85110 0.85512 0.81900
energy (hartree)
bond length (A)
1.181712 1.318607 1.595363 0.997703 1.003946 1.016251 1.022335 1.022335
1.168083 1.321124 1S35483 0.993893 0.9 98989 1.010022 1.013536 1.013535
angle
bond order
1.90016 1.05015 0.57243 0.80567 0.78393 0.77916 0.76343 0.7 6343
1.95924 1.15812 0.69202 0.86041 0.84346 0.83173 0.8 1728 0.81728
atomic charge
energy (hartree) HI,
6-31G**
0 :2
bond length (A)
1.285564 1.314736 0.998280 1.000761 1.324161 0.999388
1.277374 1.310728 0.994496 0.996697 1.317122 0.995259
angle
Urea Protonation
The Journal of Physical Chemistry, Vol. 97, No. 33, 1993 8613
TABLE 11 (Continued) bond length (A)
HJVZ
HsOi 02H5 HSOZ Hi02 bond order
c
01
NIC HINI HZNI N2C H3N2 Ha2
Hs01 02Hs
H6O2
Hi02
3-21G
6-31G**
0.998 156 1.040426 1.425880 0.967083 0.965406 1.17181 1.11117 0.8 1275 0.79468 1.07331 0.80731 0.81349 0.50298 0.20902 0.75850 0.76337
0.994730 0.971034 1.685372 0.946774 0.945312 1.21801 1.23102 0.86226 0.84928 1.20143 0.85744 0.85834 0.70855 0.08534 0.84643 0.85146
3-21G
angle
HsOiC
02H501 H602HS H702H5
atomic charge
C 01
NI HI
H2 Nz H3 H4 H5 02 H6
Hi
energy (hartree) Variations in the stretching frequency of a given bond with molecular surroundings are often attributed to changes in bond multiplicity. Thus the increase in the N-C-N symmetrical stretching frequency from urea (1003 cm-l) to protonated urea (101 5 cm-l) can be attributed to an increase in the double bond character of the N-C-N linkage, whereas the decrease in the carbonyl stretching frequency from urea (- 1662 cm-1) to protonated urea (- 1640 cm-l) can be attributed to a decrease in the doublebond character of C=O linkage. The Raman shifts to higher frequency for the N-C-N symmetrical stretching and to lower frequency for the C - 0 stretching in the aqueous solutions of the protonated urea and urea-15N2are consistent with a point of proton attachment to the 0 atom (to give 0-H+ or O-H+) rather than the N atom (as -NH3+). The proton transfer to the 0 atom results in less double bond character in the C = O bond and more double bond character in the N-C-N bond. Less double bond character in the C=O bond results in the C=O stretching vibration shift to lower frequency. The greater conjugated double bond character in the N-C-N bond results in the N-C-N symmetrical stretching vibration shift to higher frequency. If urea were protonated at the N atom, the N-C-N bond lengths would be expected to increase. The N-C-N bond would have less double bond character in the protonated urea than in the neutral urea so that the N-C-N symmetric stretching vibration would shift to lower frequency. The effect of proton transfer in aqueous solutions of urea with strong monobasic acid has also been associated with a new Raman band in low-frequency hydrogen-bonded region (Figure 4) in the R Q ( w )and ~ ~ R Q ( w )plots ~ of 2.00 mo1.L-l urea and urea-15N2in 37% HCl. The polarized Raman band in the R Q ( w spectrum )~~ at about 205 cm-1 appears to be due to the symmetric stretching motion of a hydrogen bond, H~O.V+HOC(NH~)~, vibration. A similar but much weaker band has been reported35 for pure water at about 190 cm-'. A check of this region for the RQ(w)plots of water or 2.00 mo1.L-1 urea and ureaJ5N2 aqueous solutions did not reveal any significant intensity. In an effort to learn about the nature of urea protonation ab initio methods have been employed to calculate optimum geometries and minimum energies for neutral urea, nitrogen protonated urea +HsNCONH2, oxygen protonated urea +HOC(NH2)2, and oxygen protonated urea bound to a surrounding water, H20-.+H-OC(NH2)2. The results are collected in Table IIforthe3-21Gand 6-31G** basissets. Thecalculatedoptimum geometries for urea and for oxygen protonated urea were in excellent agreement with the crystal structure of urea reported by Wyckoff21-22 and the crystal structure of uronium nitrate reported by Worsham and Bu~ing.2~ Since the 3-21G minimum basis set tended to over emphasize the effect of hydrogen bonding, the present discussion refers to the results for the 6-31G** basis set. The optimum geometry of urea had all of the atoms in the
6-31G**
125.5464 17 1.83 18 116.2835 132.7224
117.0926 172.1547 114.3467 138.8464
1.33554 -0.77503 -0.92830 0.40543 0.42821 -0.93262 0.41070 0.40399 0.52562 -0.77675 0.44758 0.44928 -298.737955
1.02563 -0.58 128 -0.7 1242 0.36560 0.38427 -0.73229 0.36850 0.36803 0.46593 -0.72062 0.38838 0.38026 -300.414157
plane. This basic planar structure was retained for the various protonated forms of urea except for the N protonated urea where the NzCO skeleton remained planar but the protonated nitrogen atom had a hybrid tetrahedral sp,p,p, electronic configuration. The minimum energy for the optimumgeometry of H2NCONH3+ was -224.321 650 hartrees in the 6-31G** basis set or -223.062 906 hartrees in the 3-21G basis set; when the proton was attached at the carbonyl oxygen, the optimum geometry had a planar structure and the minimum energy for the optimized geometry of +H-OC(NH2)2 was -224.358 136 hartrees in the 6-31G** basis set or -223.097 906 hartrees in the 3-21G basis set (Table 11). These results revealed that the proton attachment point was energetically more favorable at the carbonyl oxygen not at the nitrogen of the amide group. For protonated urea, the bond length between the carbonyl oxygen and the proton was about 0.95 8, with a bond order of 0.82. In the crystal of uronium nitrate the 0-H+ bond length has been measured as 1.006 8, but hydrogen bonding to nitrate caused this bond to lengthen. In the hydrated, protonated urea discussed below the 0-H+ bond length increased to 0.971 8, because of the hydrogen bond to a water molecule. The net atomic charge of the proton was 0.41 (Table 11). These data suggested that the bond between the carbonyl oxygen and the proton, +H-OC(NH2)2, was like a normal HO bond. The ab initio calculations also indicated that when urea linked up with a proton at the oxygen atom, the bond length between C and 0 increased from about 1.202 8, (neutral urea) to 1.291 8, (protonated urea) and the correspondingbond order decreased from about 1.806 (neutral urea) to 1.152 (protonated urea), whereas the bond length between C and N decreased from about 1.360 8, (neutral urea) to 1.306 8, (protonated urea) and the relative bond order increased from about 1.022(neutral urea) to 1.258 (protonated urea). These changes indicated that the CO bond had less double bond character and the CN bond had more double bond character in the protonated urea than in the neutral urea. The ab initio results are consistent with the spectroscopic observations that on protonation the CO stretching vibration shifted to lower frequency and the NCN symmetric stretching vibration shifted to higher frequency. In aqueous solution, protonated urea will probably be hydrogen bonded to at least one surrounding water. The ab initio calculations for hydrated, protonated urea confirmed that there was a strong interaction between +HOC(NH2)2and water. The calculated bond length and bond order between the oxygen of the hydrated water and the proton, H20-+HOC(NH2)z, were about 1.685 8, and 0.085, respectively. This strong hydrogen-bond interaction can account for the presence of the band at 205 cm-l in the low-frequency region (Figures 6 and 7) for protonated urea and protonated urea-lSN2 in HC1 aqueous solutions. For the purpose of discussion, it is reasonable to compare the calculations for urea with those of protonated, hydrated urea.
8614 The Journal of Physical Chemistry, Vol. 97, No. 33, 1993
TABLE III: Relative Integrated Intensities of 1.000 m0l.L-1 Urea with Added HCP
Wen and Brooker
TABLE V Results of 1.OOO m0l.L-1 Urea with Added HCf Aaueous Solutions' Iiois
C-.H,O+
= (CW0H@)/ (CH,O+~-)
0.013 104 0.132 64 0.237 06 0.365 85 0.42481 0.499 49 0.567 80
0.0643 0.2891 0.4934 0.6264 0.7115 0.8069 0.9063
1.92 1.93 1.92 1.92 1.91 1.91 1.90
c0,HCI
0.100 0.500 1.00 1.50 2.00 3.00 6.00
24812.40 25120.50 25596.70 25702.10 26134.90 26589.40 25063.50
20966.97 15795.27 11063.92 7821.15 5813.45 3473.85 878.78
0.42251 0.31439 0.21612 0.15215 0.11122 0.065324 0.017531
650.28 6663.97 12135.91 18806.23 22204.73 26562.28 28462.11
0.013104 0.13264 0.23706 0.36585 0.42481 0.49949 0.56780
a Where A934 is the integrated intensity of 0.5000 mol-L-I cio4- and I1003and I1015 areintegratedintensitiesofurea andprotonated urea relative
to the internal standard of 1.000 mo1.L-l Clod-, respectively.
TABLE I V Relative Integrated Intensities of Aqueous Solutions of Urea with 0.5000 m0l.L-l NaCIO4 as Internal Standard' Cur, (mold-,) A934 A1003 I1003 0.5000 0.7500 1.ooo 1.250 1so0 1.750 2.000
251 65.00 252 24.30 249 45.00 246 70.30 249 29.30 244 34.50 236 43.40
104 00.20 159 37.10 217 42.00 279 67.20 339 87.10 407 41.80 448 13.00
0.206 44 0.315 91 0.435 80 0.566 82 0.681 67 0.833 69 0.947 69
Where A934 and A1003 are integrated intensities of 0.5000 mol-L-l Clod-and urea, respectively,I1003 is the integrated intensity of urea relative to the internal standard of 1.000 mo1.L cio4-. When +HOC(NH2)2 was hydrated to water, the bond length between 0 and C increased from about 1.202 A (neutral urea) to 1.277 A (hydrated, protonated urea) and the corresponding bond order decreased from about 1.806 (neutral urea) to 1.218 (hydrated protonated urea); the bond length between C and N decreased from about 1.360 A (neutral urea) to 1.31 1 (hydrated, protonated urea) and the corresponding bond order increased from about 1.022 (neutral urea) to 1.231 (protonated urea with a surround water). Again the geometry changes were in agreement with the deductions of the spectroscopic measurements. The optimum geometry of hydrated, protonated urea, H20+HOC(NH2)2, would seem to be a better representation for aqueous, protonated urea than the just protonated urea, +HOC(NH2)2. In the following discussion the HzO-.+HOC(NH~)~ has been considered to be the most probable form of protonated urea in aqueous solution. Raman spectra of 1.000 mo1.L-I urea in 0.100, 0.500, 1.00, 1.50, 2.00, 3.00, and 6.00 mo1.L-1 HCl aqueous solutions were recorded between 850 and 1100 cm-1. The above aqueous solutions were prepared with added NaC104 at 0.5000 mo1.L-l as an internal intensity standard. The use of the 934-cm-1 band of the C104- an internal intensity reference has permitted quantitative studies of the effect of concentration on band intensities. The results for the 1.000 mol-L-1 urea with added HCl have revealed that the relative intensities of bands due to neutral urea (e.g., 1003 cm-1) decreased with the increased concentration of HCl and the relative intensities of bands due to protonated urea (e.g., 1015 cm-1) increased with the increased concentration of HC1 (Figure 5 ) . The relative integrated intensities of urea (1003 cm-1) and protonated urea (1015 cm-I) have been listed in Table 111. In order to obtain the molar scattering factor of neutral urea, Jlm3, the Raman spectra of aqueous solutions of 0.5000,0.7500, 0.1000, 1.250, 1.750, and 2.000 mo1.L-I urea were recorded (Figure 6). A 0.5000 mol-L-1 quantity of NaC104 was added to each solution as a internal standard. The corresponding relative integrated intensities of the 1003-cm-1 band have been listed in Table IV. The molar scattering factor of urea, Jim3 = 0.481 f 0.024 Lsmol-l, was obtained by using a computer software (Minitab Release 7.2) least squares fit for the concentrations and corresponding relative integrated intensities.
Qc
(mo1.L-9
I1003
0.100 0.500 1.00 1.50 2.00 3.00 6.00
0.422 51 0.314 39 0.216 12 0.152 15 0.111 22 0.065 324 0.017 5 3 1
0.9357 0.7109 0.5066 0.3726 0.2885 0.1931 0.0937
Cur, and Curoa.~,0+ calculated from the equations I1003 = -0.02754
+ 0.4810 Cur,, and C u r t a ~ I=~CO,~,, + - Cum, respectively.
TABLE VI: Data of 1.000 m0l.L-I Urea with Added HCI Aqueous Solutions'
0.100 0.500 1.00 1.50
0.0356 0.211 0.507 0.874
0.287 0.285 0.284 0.284
2.00 3.00 6.00
1.29 2.19 5.09
0.282 0.280 0.279
CBH+ and CBare concentrationsof protonated urea and neutral urea, respectively. 1. 0
1.
0. 0
b
1. 0
2. 0
3. 0
HCL
*. 0
5. 0
6. 0
LMOL/LI
F i p e 7 . Populationsof neutral urea and protonated urea in 1.000mo1.L-1 for different concentrations of HC1 (a) neutral urea and (b) protonated urea.
With theknown Jim3 valueof urea, theconcentrationsof neutral urea and protonated urea in 1.000 mol.L-* urea with added HCl aqueous solutions were calculated. The relative integrated intensities and calculated concentrations are collected in Table V. The concentrations of protonated urea were calculated from the relationship
where C u r m ~ 3 ~and +(w Cur- are theconcentrations of hydrated, protonated urea and neutraTurea in the solutions, respectively; CO,,,~=(* is the initial concentration of urea. In this case, the initial concentration of urea was maintained at 1.OOO mo1.L-1. From the concentrations and relative integrated intensities the molar scattering factor of protonated urea with a surrounding water, J l o l s = 0.639 f 0.032 L-mol- was obtained by a least squares fit. The results indicated that the molar scattering factors of protonated urea with a surrounding water, Jlol5, and neutral urea, J1@~3, are not identical but that JlOl5 > Jlm3. Figure 7 shows the populations of neutral urea and protonated urea in 1.OOOmo1.L-I urea with added HCl aqueous solutions. For 1.OOO mo1.L-1 urea with 1.00 mo1.L-l HCl aqueous solution, the concentrations of neutral urea and protonated urea are almost
Urea Protonation
The Journal of Physical Chemistry, Vol. 97, No. 33, 1993 8615 and it follows that
where
rearranging
Figure 8. Possible structures for the urea dimer.
equal. Thereafter, the concentration of protonated urea does not proportionally increase with the HCl concentration increment (Figure 7). Cristinizianoand co-workers36have suggested that urea might be present in aqueous solution in the form of two possible urea dimers (Figure 8). For urea dimer structurea, further protonation would not be expected because of the lackof an availablecarbonyl center. For urea dimer structure b, just one potential carbonyl center is available, Le., two urea molecules protonate with only one H+ ion. In the present study there was no evidence to suggest the presence of urea dimer. No new bands were observed that could be characteristic of a urea dimer over a wide range of concentrationsof urea. The fact that the value of J l w ~for urea was constant suggested a single urea species. The quantitative analysis of the intensity was therefore based on protonation of urea monomers. A monoprotonation process of urea was suggested as follows in eqs 3 and 4. The concentration of H30+(aq),CH~O+(*, can be
A plot of the quantity determined by the left-hand side of eq 11 versus C H + has ( ~ an intercept -PKA (the thermodynamic value) and the slope a can be used to construct an acidity function for the acid-substrate pair. The values of pH + loglo CBH+(~/CB(,! and CH+(@ for 1.00 mol-L-' urea with added HCl have been listed in Table VI. A least squares fit from the values of pH + loglo CBH+,/CB, and CH+(*gave thevalues Of -pKA = 0.285 and a = -0.00141. Thus one obtains K A = 1.93. This value is very close to the value, K A = 1.92 L-mol-', obtained from the simple interpolation. Therefore the equilibrium constant of urea monoprotonation can be given a value of 1.93 with an uncertainty of about 5%.
4. Conclusion From this study, urea was found toprotonate in strong aqueous acid. The Raman observationsled to the conclusion that proton transfer for aqueous solutions of urea with strong monobasic acid, e.g., hydrochloricacid, occurred at the carbonyl oxygen not the nitrogen of the amide group. Theoretical calculations were consistent with this conclusion. The process of urea protonation has been treated as an acid-base equilibrium with a measured equilibrium constant of 1.93 f 0.10 L-mol-1.
References and Notes
calculated from relationship 5. cHIO+(w
cO,HIO+(w
- curea.HIO+(w
(5)
where CO,H+(* is the concentration of H+(aq)at the initial state. The concentration quotient, Qc, was calculated for each aqueous solution from the data in Table V. The results indicated that the concentration quotient, Qc, approaches a constant value, 1.92 L-mol-l. It was obvious that the activity coefficient quotient, Q, 1. Evaluationof the thermodynamicequilibriumconstant, KA,in concentrated acid can also be evaluated by the treatment which follows."
-
(7) The ratio ( Y B H + ( ~ ) / ( Y H + ( * ) should be close to unity, so the major effect of the acid concentration is the effect on YB(*. For neutral molecules a good approximation is log(YB(w) = acH+(d
(8)
(1) Davies, M.; Hopkins, L. Trans. Faraday. Soc. 1957,53, 1563. (2) Spinner, E.Spectrochim. Acta 1959,IS, 95. (3) Redpath, C.R.; Smith, J. A. S . Trans. Faraday Soc. 1962,58,462. (4) Klotz, I. M.; Franzen, J. S.J . Am. Chem. Soc. 1962,84,3461. (5) Walrafen, G. E. J . Chem. Phys. 1966,44,3726. (6) Kresheck, G.; Klotz, I. M.Biochemistry 1969,8, 8 . (7) Barone, G.; Rizzo, E.; Vitagliano, V. J . Phys. Chem. 1970,74,2230. ( 8 ) Finer, E.G.; Franks, F.; Tait, M. J. J . Am. Chem. Soc. 1972,94, 4424. (9) Barone,G.; Castronuovo,G.; Volpe, C. D.; Elia,V. J. Solution Chem. 1977,6,117. (10) Kobayashi, M.; Nishioka, K. J . Phys. Chem. 1987,91,1247. (11) Lovas, F. J.; Suenram, R. D.; Fraser, G. T.; Gillies, G. W.; Zozom, J. J . Chem. Phys. 1988,88,722. (12) Kresheck, G. C. J . Phys. Chem. 1969,73,2441. (13) Konicek, J.; WadsB, I. Acta Chem. S c a d . 1971,25, 1541. (14) Savage, J. J.; Wood, R. H. J . Solution Chem. 1976,5, 733. (15) Ojelund, G.; Skdd, R.; Wads6, J. J . Chem. Thermodyn. 1976,8,45. (16) de Visser, C.; Pel, P.; Somsen, G. J. Solution Chem. 1977,6, 571. (17) Leslie, T. E.;Lilley, T. H. Biopolymers 1985,24, 695. (18) Grolier, J. P. E.; Spitzer, J. J.; Wood, R. H.; Tasker, I. R. J. Solution Chem. 1985,14, 393. (19) Della Gatta, G.; Barone, G.; Elia, V. J. Solution Chem. 1986,15, 157. (20) Barone, G.; Castronuovo, G.; Vecchio, P. D.; Elia, V.; Giancola, C. Thermochim. Acta 1987,122, 105. (21) Wyckoff, R. W. G. Z . Kristallogr. 1932,81,102. (22) Wyckoff, R. W. G. 2.Kristallogr. 1934,89,462. (23) Finar, I. L. Organic Chemistry, 6th ed.;Longmans: London, 1973. (24) Worsham, J. E., Jr.; Busing, W. R. Acta Crysrallogr. 1969,B25,572. (25) Brooker, M. H.;Faurskov Nielsen, 0.; Praestgaard, E. J . Raman Spectrosc. 1988,19,71. (26) Murphy, W. F.; Brooker, M. H.; Faurskov Nielsen, 0.;Praestgaard, E.; Bertie, J. J. Raman Spectrosc. 1989,20, 695.
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The Journal of Physical Chemistry, Vol. 97, No. 33, 1993
(27) Poirier, R. A., Chem. Dept., Memorial University of Newfoundland, St. John’s, Newfoundland, Canada AlB 3x7,private communication. (28) Hugus, Z.Z.,Jr.; El-Awady, A. A. J. Phys. Chem. 1971,75,2954. , (29) Keller, W. E. J. Chem. Phys. 1948,16, 1003. (30) Waldron, R. D.; Badger, R. M. J . Chem. Phys. 1950,18,566. (31) Kumler, W. D.;Fohlen, G.M. J . Am. Chem. Soc. 1942,64,1944. (32) Andrew, E. R.; Hyndman, D. Discuss. Faraday Soc. 1955,19,195. (33) Stewart, J. E. J. Chem. Phys. 1957,26,248.
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