J . Phys. Chem. 1990, 94, 4015-4025 TABLE 111: Atmospheric Photodissociation Rate Constants for Formaldehvde 1OSk,ns-I zenith IO nm 0.5 nm angle, 0 10
20 30 40 50 60 70 I8 86
3.14 3.09 2.94 2.67 2.29 1.80 1.22 0.624 0.242 0.041
4.79 4.74 4.57 4.27 3.82 3.19 2.31 1.38 0.613 0.127
3.25 3.20 3.03 2.75 2.35 1.84 1.24 0.624 0.237 0.0383
3.99 3.95 3.80 3.55 3.17 2.64 1.95 1.13 0.499 0.1023
nuncertainties are estimated to be about 1-2%, on the basis of the least-squares fits to the cross-section data and an estimate of systematic errors in the pressure measurements. co-workers.5~9These quantum yields gave the same values as those recommended by DeMore et al. (1987) when recalculated at a resolution of I O nm. The calculated photodissociation rate constants are listed in Table I l l . To compare our results with previous calculations, we also calculated the photodissociationrate constants using IO-nm averages for the cross sections, actinic flux, and quantum yields as a function of wavelength. These results are also included in Table 111. We found that the resolution of the cross sections used
4015
in the calculations had a significant effect only on the calculated photodissociation rate constants for the & channel (H, + CO). For example, at a zenith angle of Oo, the photodissociation rate constant for I#I~was 17% lower using the 0.5-nm averaged cross sections rather than the IO-nm averaged cross sections, but the photodissociation rate constant for increased by only 3%. The effect of the increased resolution of 0.5 nm in the calculations is to decrease the photodissociation rate constant for c#I~,while the effect of the larger cross sections measured here is to increase this photodissociation rate constant. Figure 4 compares our results with those calculated from the cross sections recommended by DeMore et aL7 Our values for the photodissociation rate constant for channel 41varies from 8% larger than the recommended value at a zenith angle of 0' down to 2% lower at a zenith angle of 86'. Our values for the photodissociation rate constant for channel 4, are about 9-1 1% lower than the recommended values. More free radicals (HO,) will be formed in model calculations of atmospheric chemistry using our new photodissociation rate constants, since we find that more formaldehyde photolyzed into the H HCO channel than was previously thought. Although detailed atmospheric modeling is required to determine the exact effect on ozone of these results, the general effect will be to increase the calculated amount of ozone formed.
+
Supplementary Material Available: Appendix consisting of a table listing UV absorption coefficients averaged every 0.5 nm (6 pages). Ordering information is given on any current masthead page.
Resonance Raman Studies of Guanidinium and Substituted Guanidinium Ions Roseanne J. Sension, Bruce Hudson,* Department of Chemistry, University of Oregon, Eugene, Oregon 97403
and Patrik R. Callis Department of Chemistry, Montana State University, Bozeman. Montana 5971 7 (Received: October 10, 1989)
Resonance Raman spectra of aqueous solutions of guanidinium, methylguanidinium, 1,l -dimethylguanidinium,ethylguanidinium, and L-arginine salts are reported. The results are interpreted in terms of excited electronic state symmetries and geometry changes with the help of semiempirical INDO/S calculations. The low-lying E'(mr*) excited state of guanidinium exhibits a linear Jahn-Teller distortion along the e' CN, stretching nuclear coordinate. The resonance Raman spectra also indicate a significant force constant change, possibly even a double minimum potential, along the CN, umbrella coordinate in the excited electronic state. The ?r s* transitions of guanidinium and of all of the substituted guanidinium ions involve charge transfer between the nitrogen atoms and the central carbon atom. In the ground state, the positive charge is shared by the carbon atom and all three of the nitrogen atoms. In the excited electronic state, the carbon atom has a net negative charge. The resonance Raman spectra of all of the substituted guanidinium ions are characterized primarily by a strong enhancement in the intensity of a band located at approximately 1650 cm-' (1620 cm-' in the deuterated species). This band is assigned to the fundamental vibrational transition in the asymmetric CN, stretching coordinate involving the symmetric C-N stretching of the unsubstituted nitrogen atoms.
-
Introduction Guanidinium ion, (C(NHJ3)+, is a simple 10-atom species that has been of much interest to biochemists due to its presence as the functional group of the amino acid arginine and its presence as a constituent in other biologically active molecules, and because of its use as a protein denaturant. This species has, however, received only scant attention from spectroscopists since the late 1930s. There are relatively few references in the literature on the spectroscopy of this ion for reasons that can be inferred from its properties. Because the ion only exists in aqueous solution (or in other polar solvents) or in the crystalline form, it is not very amenable to infrared (IR) studies, although there are a few IR studies of guanidinium crystals reported.Is2 Water does not 0022-3654/90/2094-4015$02.50/0
interfere with Raman spectroscopy and there are several Raman studies of guanidinium reported in the literature, most dating from the late 1 9 3 0 ~ . ~ The - ~ electronic spectrum is also of interest because of the high symmetry of guanidinium and the simplicity of its orbital pattern. Unfortunately, guanidinium does not begin ( 1 ) Angel], C. L.; Shepard, N.; Yamaguchi, A.; Shimanouchi, T.; Miyazawa, T.; Mizushima, S. Trans. Faraday SOC.1957, 53, 589. (2) Bonner, 0. D.; Jordan, C. F. Specfrochim. Acta 1976, 32, 1243. (3) Edsall, J. T. J . Phys. Chem. 1937, 41, 133. (4) Otvos, J. W.; Edsall, J. T. J . Chem. Phys. 1939, 7, 632. (5) Gupta, J. J . Indian Chem. Soc. 1936, 13, 515. (6) Bonner, 0. D. J . Phys. Chem. 1977, 81, 2247. (7) Ananthakrishnan, R. Proc. Indian Acad. Sci. A 1937, 5 , 200.
0 1990 American Chemical Society
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The Journal of Physical Chemistry, Vol. 94, No. 10, 1990
Sension et al.
I
I
H
Ib
I.
N 0H II
I1
II I.
ti
H II
I
H
H Ill
..
I
I
H
H IV
I.
Id
IC
”,
0C
N
I
\N/ H
I
H
H V
I
H
H
H
H
H
H V4
Figure 1. Guanidinium ions and resonance structures. Ia is the guanidinium ion, Ib-Ie are the four resonance structures contributing to the ground electronic state of the guanidinium ion. I1 is guanidine, 111 is methylguanidinium, IV is l,l-dimethylguanidinium,V is ethylguanidinium, and VI is the L-arginine ion.
to absorb until about 200 nm. The peak of the first absorption band lies below I90 nm. Thus the electronic structure of guanidinium in solution could not be studied by using conventional far-ultraviolet absorption techniques. Several theoretical studies of guanidinium have been reported in the literature. These have dealt primarily with the torsional barrier for N H I rotation, or with the hydrogen-bonding interaction of guanidinium with carboxylate or phosphate.*s9 None of these theoretical studies include a discussion of the excited electronic states of guanidinium. Despite the experimental difficulties, the spectroscopy of guanidinium ions is of considerable interest. Through spectroscopic, crystallographic, and theoretical studies, the guanidinium symmetry, cation has been determined to be planar, with D3,, consistent with an electronic structure described by the four resonance structures shown in Figure 1 . ’ ~ ’ ~ The “Yde1ocalization”l’ of the guanidine group (11) and of the guanidiniuni ion are of importance in the stabilization and function of a number of molecules of biological importance. These include the amino acid arginine, creatine, and many purines, including the nucleic acid base, guanine. From a purely chemical physics point of view, the ion is of interest due to the resonance stabilization of the x-system, and the “Y-aromatic” character of guanidinium which results in a stabilization comparable to that of cyclic aromatic molecules such as benzene.)’ In this paper we present a resonance Raman study of guanidinium and several substituted guanidinium ions. Although the absorption spectrum of guanidinium does not give much information on the excited electronic states, resonance Raman spectra provide information on the excited-state symmetry and geometry through selective enhancement of vibrational bands as a function of excitation wavelength. The work presented in this paper is rnotivated by two separate considerations. The first consideration is the continuation of the investigation of m * systems being carried The 3-fold symmetry of C(NH2)3+ out i n this ~
(8) Kollman. P.: ‘McKelvey. J.: Gund, P. J . Am. Ckem. SOC.1975, 97, 1640. (9) Capitani, J. F.; Pedersen, L. Chem. Phys. Letr. 1978, 54, 547. Sapse, A. M.: Massa, L. J . J . Org. Chem. 1980, 45, 719. Cotton, F. A.; Hazen, E. E. Jr.; Day, V . W.; Larsen, S.; Norman, J. G.: Wong, S. T. K.; Johnson, K . H. J . Am. Ckem. SOC.1973, 95, 2367. Herzig, L.; Massa, L. J.; Santoro, A,; Sapse, A. M. J . Org. Chem. 1981, 46, 2330. Sapse, A. M.; Russell. C. S. In?. J . Quantum Chem. 1984, 26, 91. ( I O ) Pauling, L. The Nature of the Chemical Bond, 3rd ed.: Cornell University: Ithaca, NY, 1960; p 286. ( 1 1 ) Gund, P. J . Chem. Educ. 1972, 49, 103. ( 1 2 ) Ziegler, L. D.; Hudson, B. J . Chem. Phys. 1983, 79, 1197. (13) Sension, R. J.; Hudson, B. J . Chem. Phys. 1989, 90, 1377. (14) Ziegler, L. D.; Hudson, B. J . Chem. Phys. 1981, 74, 982. Gerrity, D. P.: Ziegler. L. D.: Kelly. P. B.; Desiderio, R. A,; Hudson. B. J. Ckem. Phys. 1985. 83. 3209. ( 1 5 ) Sension, R. J.: Brudzynski, R. J.: Hudson, B. Manuscript in prepa-
ration.
results in degenerate excited electronic states that will be subject to Jahn-Teller splitting. The electronic structure of guanidinium is calculated to be (our calculation and ref 8)
The filled e” a H O M O and the a;’ a * L U M O imply that the lowest lying excited electronic state will be degenerate. The resulting Jahn-Teller splitting is expected to give rise to interesting and novel resonance Raman spectra. In addition to the resonance Raman studies, the excited electronic state energies, symmetry assignments, and geometry changes for guanidinium and methylguanidinium and 1,l -dimethylguanidinium are further investigated by using semiempirical INDO/S calculations. An estimate is made of the magnitude of the Jahn-Teller effect in the lowest lying excited electronic state of guanidinium and of the magnitude of the splitting of this state associated with methyl substitution. These calculations, which include single and double excitation configuration interaction, demonstrate the simplicity of the electronic spectrum of the guanidinium cation. The second motivating factor for the work presented in this paper is the study of guanidinium and substituted guanidinium ions as a key for understanding the resonance Raman spectrum of arginine, for identifying bands characteristic of the guanidinium functional group, and as background for a study of the effects of ion pairing on the vibrational spectrum of the guanidinium functional group. In order to better understand the resonance Raman spectrum of arginine, this paper will compare and contrast the spectra obtained for guanidinium, methylguanidinium. ethylguanidinium, and arginine. This is part of a series of resonance Raman studies of protein components being carried out in this laboratory.)*
Experimental Section The Raman spectrometer used for the experiments discussed in this paper has been described in detail p r e v i o u ~ l y ,and ’ ~ ~only ~~ (16) Chadwick, R. R.; Gerrity, D. P.; Hudson, B. S. Chem. Phys. Lett. 1985, 115, 24. Strahan, G. D. Ph.D Dissertation, University of Oregon, 1989. (17) Brudzynski, R. J.; Hudson, B. J . A m . Chem. SOC.,in press. (18) Mayne, L. C.; Ziegler, L. D.; Hudson, B. J . Phys. Chem. 1985,89, 3395. Mayne, L.; Hudson, B. J . Phys. Chem. 1987, 91, 4438. Hudson, B. S.; Mayne, L. C. In Biological Applications of Raman Spectroscopy; Spiro, T. G., Ed.; Wiley: New York, 1987; Vol. 2, p 181. Hudson, B.; Mayne, L. Methods Enzymol. 1986, 130, 33 1. (1 9) Hudson, B.; Sension, R. J. In Vibrational Spectra and Structure; Bist, H. D., Durig, J . R., Sullivan, J. F., Eds.; Elsevier: Amsterdam, 1989; Vol. 17A, p 363. (20) Hudson, B.; Kelly, P. B.; Ziegler, L. D.; Desiderio, R. A.; Gerrity, D.
P.; Hess, W.: Bates, R. In Advances in Laser Spectroscopy; Garetz, B. A., Lombardi, J. R., Eds.: Wiley: New York, 1986; Vol. 3, p 1.
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4017
Guanidinium and Substituted Guanidinium Ions TABLE I: Summarv of the Raman Swctra of IC(NH,)J+"
Raman shift, cm-'
416
425 536 810
11
1012
I00
1120
1470 15756 1 670b 2020 2475 2586
-2 -2vb - 1 vb
342 -3 vb 8 -3 vb 100 -3 vb 3
309
excitation wavelength, nm 240 223
-2 D
266 7D 7D
100 P
100 P
100
100
5P -7Dvb -7Dvb
10 -8vb
-5vb 1
2
-3vb
-5vb
-3vb
55vb
-6vb
218
204
200
192
100
100
100
loop
11
14
26
-5vb
-9vb
19 b
-9vb 1
30 10
37 P 34 D 31 D 15 P 12 P 6D 6D
2693
35 4OC 41
32 19 16
18
This table summarizes the observed Raman spectra. The numbers represent estimates of the relative intensities, where the intensity is assumed to be proportional to the peak height times the full width at half-maximum. In every case the intensity is normalized to the 1012-cm-I peak. For the experiments that included polarization analysis, P indicates a polarized band and D indicates a depolarized band. The notation vb indicates a particularly broad band. bThe 1575- and 1670-cm-I bands are difficult to measure for wavelengths longer than 204 nm because of interference from the H,O bending vibration at 1650 cm-I. The H,O peak is a broad band and along with these two guanidinium bands results in the very broad peak observed between 1550 and 1700 cm-'. (See for instance the 342-nm spectrum in Figure 2.) CInthe 192-nm spectrum, the 1558-cm-I O2 vibration is observed with considerable intensity. This results in much uncertainty in the intensity of the 1575-cm-I guanidinium band.
a general description will be given here. The stimulated Raman scattering of the second, third, and fourth harmonics of a Quanta Ray Nd:YAG in high-pressure H2 gas (approximately 120 psig) provides the necessary Raman excitation wavelengths. The output of the Raman shifting cell is dispersed by a quartz prism and the desired excitation wavelength is focused on the sample by using a quartz lens. The back-scattered radiation is collected and collimated by an f/1 quartz collection lens and focused on the slit of the monochromator (ISA HR-640) by a lens which matches the f-number of the monochromator. When polarization information is desired, a quartz stacked plate polarization analyzer is placed between the collection lens and the focusing lens. The sample solutions were circulated through the sampling region by using a small Teflon positive displacement pump (Fluorocarbon Saturn Teflon pump Model SPM 100). In the sampling region, the solution stream is directed through a guided flow device designed to produce a flat liquid surface.21 The use of a flat surface aids in stray light rejection by producing a well-defined reflected beam. The samples were prepared by dissolving guanidine hydrochloride ( M C B reagents), guanidine sulfate (Kodak), 1methylguanidine hydrochloride (Aldrich), 1-methylguanidine sulfate (Kodak), 1 ,]-dimethylguanidine sulfate (Aldrich), 1ethylguanidine hydrochloride (Aldrich), or L-arginine hydrochloride (Alfa Products) in twice distilled HzO. Sample concentrations ranged from 5 X to 1.5 M. No significant changes in the preresonance or resonance Raman spectra as a function of concentration or counterion were observed. N-deuterated samples of guanidine hydrochloride, guanidine sulfate, 1-methylguanidine sulfate, and 1, I-dimethylguanidine sulfate were prepared by dissolving the crystalline hydrogenated compound in D 2 0 (Cambridge Isotope Laboratories, isotopic purity 99.9%) and then recrystallizing the sample by evaporating the solvent. This procedure was repeated several times. As most of the Raman spectra were obtained by using a free-flowing solution, the sample would pick up hydrogen atoms from atmospheric water during the course of each experiment. In many cases, the collection of hydrogen atoms could be monitored by the appearance of the 0-H stretching band of water. A comparison of the spectra obtained by using fresh sample with the spectra obtained by using sample contaminated with hydrogen was made for several excitation wavelengths. For low hydrogen concentration the contamination resulted in a broadening of some of the vibrational bands, most notably the 920-cm-I CN, breathing mode and the 1278-cm-' N H 2 scissors vibration of g~anidinium-d~. It did not result in the appearance of any new peaks or in an observable intensity change in any of the guanidinium-d6 bands. (21) Harhay, G.;Hudson, B. Manuscript in preparation
c
14000
1
I
c
.g 12000 E
g
0
c
10000
8000
.-0 g 6000 .-c w
4000
3
2000 0.0 190
200
210
220
230
240
250
Wavelength (nm) Figure 2. Absorption spectra of guanidinium (G), 1-methylguanidinium (MG), I-ethylguanidinium (EG), L-arginine hydrochloride (A), and 1,l-dimethylguanidinium (DMG), in the region of the resonance Raman spectra. The arrows indicate the excitation wavelengths used in the resonance Raman experiments. TABLE 11: Summary of the Raman Spectra of IC(NDAl+" excitation wavelength, nm Raman shift, cm-' 342 223 218 204 200 455 15 D
630 920 1278 1470
1600 1840
4D 1OOP 12P 2
100 14 13 vb 12 3
1OOP 13P 11 P 17 D 2
192 100 15 40
35
1OOP lOP 38 P 37 D
8
8
17
100
14 40vb
49
"This table summarizes the observed Raman spectra. The numbers represent estimates of the relative intensities, where the intensity is assumed to be proportional to the peak height times the full width at half-maximum. In every case the intensity is normalized to the 920cm-l peak. For the experiments that included polarization analysis, P indicates a polarized band and D indicates a depolarized band. The notation vb indicates a particularly broad band. The 342-nm spectra of all of the compounds were measured by using a sealed quartz cell, and thus there was no contamination problem in these spectra. The absorption spectra and molar extinction coefficients of guanidinium and the substituted guanidinium ions were measured with a Varian DMS 300 UV-visible spectrophotometer. The spectra obtained are shown in Figure 2. The measured molar extinction coefficients should be accurate to two significant figures. Results Guanidinium. W e have obtained Raman spectra of guanidinium and guanidinium-d6 using a large number of excitation wavelengths. The resonance Raman spectra and representative
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The Journal of Physical Chemistry, Vol. 94, No. 10, 1990
TABLE 111: Calculated Excited X. nm u.. cm-’ 187 53 400 146 68 400 115 86 800 1 I4 87 500 106 100
94 100 99 550
Sension et al.
Electronic States of [C(NH2),JC symmetry D,, oscillator strength E’(re’’,r*a;’) 0.58 E’(d’,u*a,’) 0.00 A2)’(re’’,u*e’) 0.18 E”( re”, u *e’) 0.00 (re”,u*a ,’) (ra,”,u*e’) A ”(re”, u* e’) 0.00 E”(re”.u*a,’) 0.00
,
off-resonance and preresonance spectra are shown in Figures 3 and 4. The excitation wavelengths below 250 nm are indicated in Figure 2 . The results are summarized in Table I for guanidinium-h6 and Table 11 for guanidinium-d6. The results of a semiempirical INDO/S calculation of the ground and excited electronic states of guanidinium including single and double excitation configuration interaction are shown in Table 111. The C-N bond lengths were set at 1.33 A, the C-H bond lengths were set at 1.OO A, and all of the bond angles were held at 120”. The lowest lying excited electronic state is found to be the doubly degenerate E’(re’’,r*a2/l) state, v, = 53 400 cm-]. The transition from the ground electronic state to this excited electronic state is symmetry allowed with in-plane polarization. The oscillator strength cf, is calculated to be 0.58. Thus, this transition is calculated to be strongly allowed, as is consistent with the experimental molar extinction coefficient which is projected to be greater than IO4 M-I cm-’ at the peak (see Figure 2 ) . A molar extinction coefficient of 6 = IO4 M-’ cm-’ corresponds to f = 0.1 for a transition width of ca. 4000 cm-l. The next allowed electronic state is the A2”(re”,u*e’) state which is weaker cf= 0.18) and much higher in energy (v, = 86 800 cm-I). The infrared and off-resonance Raman spectra of guanidinium in the crystalline phase, and the off-resonance Raman spectrum of guanidnium in aqueous solution, have been reported several times previo~sly.’-~ The most complete vibrational analysis for guanidinium is that of Angel1 et al.’ The experimental frequencies
Li I t
N I
223
I
1000
I
2000
I
I
I
3000
I ’
4000
Raman S h i f t (l/cm)
Figure 3. Raman spectra of guanidinium sulfate or guanidinium chloride obtained by using the indicated excitation wavelengths. The bands are labeled according to the convention M,, where M is the number of the normal mode and n is the number of quanta of excitation in that mode. The initial state is assumed to be the vibrationless ground state.
and vibrational assignments are summarized in Tables IV and V . In compiling these tables, we have retained the mode numbering system and most of the peak assignments of ref 1. The numbering system used for the vibrational modes does not follow the usual convention for practical reasons. In the crystalline form guanidinium has been determined to have C3, site symmetry.’ In the reduced symmetry environment, the D3hA,’ and A T vibrations are of A , symmetry (Raman and IR allowed), the D3,, A i and A,” vibrations are of A, symmetry (inactive), and the D3h E’ and
TABLE I V Vibrational Assignments for [C(NH,),]’ experiments“ crystalline samples vibrations: sym, descripn, activity VI
Y2
u3
u4 US
sym N H 2 str N H 2 scissors C N , sym str C N , angle def NH, wag
IR a
b
722 520
d
C
e
A,’ Symmetry (Raman Active) 3338 1624b 1006
1005 724 520
aqueous solution Raman
Raman
f
g
3328 1630 1007
3394
A,“ Symmetry (Infrared Active) 730‘ 518
1012 73Ic
h
I
j
1670b 1015
1670* 1005
1012 735c
732c
A,’ Symmetry (Inactive) Vl
NH, asym str N H , rock
Va
N H 2 twist
u9
N H 2 asym str N H 2 sym str C N , deg str
3390 3320 1650
NH, deg scissors N H 2 rock C N , angle def
1550 1 I58 514
u6
A,” Symmetry (Inactive)
uI0 VI,
u12
uI3 uI4
E’ Symmetry (Raman and Infrared Active) 3396 3450 3430 3410 3300 3352 3279 3275 1670 1660 1660 1624 1660 (1640 (1643 1538 1536 I565 1554 1553 1120 525 529 522 526 526
3454 3298 1663 1569 523
1670
1660
1670
1565 1120 536
1554
1570 536
E” Symmetry (Raman Active) u15
u , ~
N H 2 twist N H , wag
830 500
500
490
533
810 425
“The experimental data comes from the following sources (Gt = C(NH,),+): (a) GtI- ref 1; (b) GtCI- ref 1 ; (c) G’CI- ref 2; (d) G+CIOc ref 2; (e) G T I - ref 7; (f) G T I - ref 2; (g) GtCIOC ref 2; (h) GtCl- ref 4; (i) G’CI- ref 2; and 6) G2’S04*- and GtC1- this work. R = Raman; IR = infrared, *This frequency may be assigned to u2 or uI, in the off-resonance spectra. However the resonance enhanced 1670-cm-’ band must be assigned to u I i on the basis of the polarization data. ‘Frequencies obtained by dividing the frequency of the approximately 1 4 7 0 - ~ m -peak ~ in half.
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4019
Guanidinium and Substituted Guanidinium Ions TABLE V: Vibrational Assignments for IC(NDA1'
experiments" crystalline samples
vibrations: sym, descripn, and
IR a
aciivity
y3
sym ND, str ND, scissors C N , sym str
y4
CN, angle def
u5
ND, wag
y6 Ul
ND, asym str ND, rock
us
ND, twist
y9
u13
ND, asym str ND, sym str C N 3 deg str ND, deg scissors ND, rock
u14
CN, angle def
y15
ND, twist ND, wag
VI
02
aqueous solution Raman
Raman b f AI' Symmetry (Raman Active)
2400 1248
2410
A; 712
2414 1257 916
g 2469 1280
h
I
j
1278 921
1275 918
1278 920
Symmetry (Infrared Active) 722
135b
{ ;;yb
