May, 1958
ABSORPTION SPECTRA OF INORGANIC AND ORGANIC COMPOUNDS OF NITROGEN
62 1
ELECTRONIC ABSORPTION SPECTRA OF INORGANIC AND ORGANIC COMPOUNDS OF NITROGEN. 11. BUTYL AND AMYL NITRITES‘ BY AUBREYP. ALTSHULLER, ISRAEL COHENAND CAROLMEYERSSCHWAB Robert A . Tafl Sanitary Engineering Center, Bureau of State Services, Public Health Service, Cincinnati 16, Ohio Received January 87, 1955
The ultraviolet absorption spectra of n-butyl nitrite, t-butyl nitrite, n-amyl nitrite and isoamyl nitrite in the 2100 to 4000 A. region have been determined in the gas state and in the following solvents: cyclohexane, isooctane, carbon tetrachloride, chlorobenzene, diethyl ether, di-isopropyl ether,,p-dioxane, ethyl acetate, acetone, a number of alcohols and water. The peak positions for n-alkyl and isoalkyl nitrites in the 30004000 d. band systems are almost completely independent of solvent. The peak positions for t-alkyl nitrites appear to show a blue shift with increasing dielectric constant of the solvent. The usual solvent effects are complicated by a possible shift in the trans-cis ratio in n-alkyl and isoalkyl nitrites. The short wave length maxima between 2050 and 2250 A. show a blue shift with increasing dielectric constant of the solvent, which may be associated with an allowed n -L ?r type electronic transition.
Although the ultraviolet absorption spectra of ther purified by passage through alumina columns to remove alkyl nitrites have been of concern in a number of peroxides and some residual aldeh~de.1~The products were for peroxides by the potassium iodide method.l6 investigations,2-1a problems remain in regard to sol- tested They were then assed through a column of Molecular Sieve, vent effects 04 peak positions and intensities in types 4A and 5& to remove water and perhaps some alcothe 3000-4000 A. region for this class of compounds. holic impurities. The short wave length peaks were determined using specFurthermore, information on the low wave length troscopic grade isooctane, cyclohexane, diethyl ether, methpeaks in alkyl nitrites is meager. The actual iden- anol, isopropyl alcohol and distilled water. The measuretification of the high and low wave length band sys- ments were made using both 10 and 1 mm. quartz absorptems with types of electronic transitions appears to tion cells. gas spectra were obtained using a one-meter gas cell be unavailable in the literature. Organic nitrites in The a dynamic system. The alkyl nitrites in an air stream also are of interest as trace components in some were passed through the cell a t about one liter per minute urban atmospheres. Consequently, the ultraviolet from a diffusion cell apparatus.16 The concentrations of absorption spectra of n-butyl nitrite, t-butyl nitrite, alkyl nitrite were of the order of several hundred parts per n-amyl nitrite and isoamyl nitrite in cyclohexane, million. Ultraviolet spectra were determined with a Cary Model 11 isoGctane, carbon tetrachloride, chlorobenzene, di- Recording Spectrophotometer. Since the band system in ethyl ether, di-isopropyl ether, p-dioxane, ethyl the 3000-4000 d. region falls in both the tungsten and hyacetate, acetone, a number of corresponding alco- drogen lamp region, the spectra were obtained with both hols and water have been measured. The gas sources. The peak positions and intensities in the 3400spectra for n-amyl nitrite and i-amyl nitrite were 3700 4.spectral region were averaged using the values from the individual determinations with each source. The peak also obtained. positions and intensities with a represenbative alkyl nitrite Experimental Details solution were checked at various mechanical slit widths and The alkyl nitrites used included n-butyl nitrite, t-butyl nitrite, n-amyl nitrite and isoamyl nitrite obtained from K and K Laboratories and Eastman Kodak Co. and the nitrites were subjected to atmospheric and vacuum distillation during various stages of the investigation. A number of the lots of alkyl nitrites were dried over calcium chloride. The stock solutions of alkyl nitrites in the solvents were prepared on a weight basis. These solutions were diluted to the appropriate concentration depending on the intensity of the absorption peak. The spectra in the 3000-4000 A. region were determined using solutions containing about 0.2% by weight of alkyl nitrites. The more intense bands at 2050-2250 d. were determined with 0.01 to 0.02% solutions,. The solvents for the ultraviolet determinations on the alkyl nitrites were of spectroscopic purity or reagent grade. The diethyl ether, di-isopropyl ether and dioxane were fur(1) Presented in part a t the Air Pollution Symposium of the 124th Meeting of the American Chemical Society, New York, New York, September, 1957. (2) W.Kuhn and H. Lehmann, Z . Elektrochem., 37, 549, 552 (1931); Z . physik. Chem., B18, 32 (1932). (3) W. Kuhn and H. Biller, ibid., B29, 1 (1935). (4) H.Elkins and W. Iiuhn. J . A m . Chem. SOC.,57, 296 (1935). ( 5 ) H. W. Thompson and C. Purkis, Trans. Faraday SOC., 32, 674 (1936). (6) C. Purkis and H. W. Thompson, ibid., 3 2 , 14G6 (1936). (7) II. W. T l ~ o n ~ ~ and s o n F. Dsinton, ibid., 8 3 , 1546 (1937). (8) P. Tarte, J. Chem. Phya., 20, 1570 (1952). (9) R. N. Haszeldine, J . Chem. Soc., 2525 (1953). (10) R. N. Hasseldine and J. Jander, ibid., 691 (1954). (11) R. N. Haszeldine and B. J. H. Mattinson, ibid., 4172 (1955). (12) H. E. Ungnade and R. A. Smiley, J . Ore. Cham., 21, 993 (1956). (13) D.H. Szuelezewski, M. Yunker and T. Higuohi, J . A m . Pharm. Assocn., 46, 776 (1958).
at the minimum scanning speed of l%l./second. No effect on peak maxima or intensities was observed as compared with the normal operating conditions. These conditions included mechanical slit widths of 0.04 mm. with the hydrogen lamp and 0.1 mm. with the tungsten lamp a t 3700 d. The mechanical slit widths in the 2050-2250 A. region ranged from 0.1 to 2 mm., but most of the values fell around 0.21 mm. In this spectral region, a 1 mm. mechanical siit width corresponds to a spectra; slit width of about 15 A. The spectra were scanned a t 10 A./second. The wave length maxima are reproducible to f 5 and are probably accurate to within 10 A. However, the short wave length peaks are quite broad and flat; consequently, these peaks are readable only to about f10 1. The molar extinction coefficients are considered to be reproducible within 2 or 3% and probably are accurate to f 5 % . The instability of the alkyl nitrites to heat and light combined with instrumental and sampIe preparation errors makes it difficult to obtain highly accurate molar extinction coefficients for these nitrites.
4.
Results and Discussion The wave length maxima and the molar extinction coefficients obtained for the four alkyl nitrites employed in various solvent systems are listed in Tables I through IV. The peaks in the 3000-4000 A. region are designated by A thr,ough G with de(14) W.Dasler and C. D. Bauer, Ind. Eno. Chem., Anal. E d s 18, 52 (1946). (15) J. Rosin, “Reagent Chemicals and Standards,” D. Van Nostrand Co., Inc., New York, N. Y., 1955,p. 179. (16) M. McKelvey and H. E. Hoelsoher, Anal. Chem., 29, 123 (1957).
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A. P. ALTSHULLER, I. COHENAND C. M. SCHWAB
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TABLE I ULTRAVIOLET ABSORPTION SPECTRA O F n-AMYL NITRITEI N VARIOUS SOLVENTS Wave lengths (d.)and molecular extinction coefficients (mole-' 1. cm.-l) Ho, b Solvent A B C D E F G 2240 Cyclohexane 3830 3700 3560 3445 3335 3235 3140 1780 28 60 74 65 46 31 25 2240 Isooctane 3830 3690 3560 3440 3330 3230 3140 1760 27 56 69 60 43 28 23 .. Carbon tctrachloride 3825 3695 3560 3440 3330 3230 3140 .. 34 73 87 76 54 36 27 .. Chlorobenzene .. 3700 3565 3445 3335 3240 3145 .. .. 78 93 81 57 37 28 2210 Diethyl ether 3840 3690 3560 3440 3330 3235 3145 1470 31 66 80 69 49 32 24 Di-isopropyl ether 3840 3690 3560 3440 3330 3230 3140 33 69 83 73 51 33 25 .. p-Dioxane 3870 3710 3570 3450 3345 3240 3140 .. 30 59 67 57 39 27 24 .. Ethyl acetate 3840 3700 3560 3445 3335 3240 3140 .. 22 46 54 45 32 22 17 .. Acetone 3880 3720 3565 3445 3340 .. .. .. 30 57 66 56 39 .. .. .. n-Amyl alcohol 3840 3695 3560 3445 3335 3235 3145 .. 27 58 70 61 44 28 21 2100 .. .. .. Water 3860 3710 3570 3460 a H peak at 222 mp in gas. Peaks in 3000-4000 A. region for n-amyl nitrite as gas a t 3840, 3680, 3560, 3430, 3330, 3220 and 3130 d. H peak at 2140 d. in methanol and a t 2220 A. in isopropyl alcohol.
TABLE I1 ULTRAVIOLET ABSORPTIONSPECTRA OF ISOAMYL NITRITEIN VARIOUS SOLVENTS Wave lengths (d.)and molecular extinction coefficients (mole-' 1. cm.-l) Solvent A B C D E F G Cyclohexane 3840 3690 3560 3440 3330 3230 3135 33 68 83 72 51 34 29 Isooctane 3830 3690 3560 3440 3325 3230 3140 26 59 74 64 45 28 19 Carbon tetrachloride .. 3690 3550 3410 3330 3230 3140 .. 75 90 78 57 36 31 Chlorobenzene .. 3690 3560 3440 3335 3235 3145 .. 82 08 85 60 39 30 Diethyl ether 3840 3700 3560 3445 3335 3235 3145 32 66 79 68 48 32 24 Di-isopropyl ether 3840 3605 3560 3440 3335 3135 3140 33 68 81 70 50 33 26 p-Dioxane 3850 3720 3580 3450 3345 3245 3150 33 64 70 57 40 25 20 Ethyl acctate 3850 3700 3565 3445 3340 3240 3145 29 56 68 58 41 27 20 Ace tone 3860 3705 3570 3445 3335 3220 .. 33 63 72 GO 42 34 .. Isoamyl alcohol 3830 3695 35GO 3440 3335 3230 3140 33 70 84 73 52 35 26 Water 3860 3710 3575 3455 3350 .. .. H peak a t 222 mp in gas. b H peak at 2110 A. in methanol and 2190 A. in 1-propanol.
creasing wave length. The A peak is often only a shoulder on the B peak. The G peaks and sometimes the F peaks are too weak t o locate them definitely. However, in many of the solutions, seven wave length yaxima are clearly identifiable in the 30004000 A. region. The single broad peaks which appear in the 2050-2250.8. region are designated as H peaks. Owing to the very poor transmission of most solvents in this region, wave length maxima could be obtained only in cyclohexane, isooctane, diethyl ether, methanol, isopropyl alcohol
H a ,b
2240 1960 2235 1810
..
.. .. .. 22 10 1610
.. ..
.. .. ..
.. .. .. 2275 1460 2080
and water. The butyl and amyl alcohols are not sufficiently transparent in this region. Lower alcohols such as methanol and isopropyl alcohol, which can be purified sufficiently for use in the 2050-2250 A. region were used for qualitative purposes, but not for quantitative determinations because of transesterification resulting in appreciable conversion of the nitrites present to methyl and isopropyl nitrite. The solvent effects on the wave length niaxima of the t-alkyl nitrites appear to be different from
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ABSORPTION SPECTRA OF INORGANIC AND ORGANIC COMPOUNDS OF NITROGEN TABLE I11 ULTRAVIOLET ABSOFPTION SPECTR.4 O F n-BETYL NITRITE IS VARIOUS SOLVENTS Wave lengths (A.) and molecular extinction coefficients (mole-' 1. cm.-l) A
Solvent
Cyclohexane
3840 32 3840 30 3830 33
Isooctane Carbon tetrachloride
..
Chlorobenzene
..
Diethyl ethcr
3840 29 Di-isopropyl ether . 3840 34 p-Dioxane 3880 31 Ethyl a.cetrtte 3850 30 Acetone 3880 28 n-Butyl alcohol 3840 28 Water 3860 H peak a t 2150 A. in methanol and
B
D
C
3695 3560 3440 70 85 74 3695 3560 3440 68 82 72 3695 3560 3440 69 82 72 3690 3560 3445 70 84 73 3700 3560 3445 67 81 70 3690 3560 3440 69 84 73 3710 3570 3450 58 65 54 3700 3565 3445 59 70 59 3720 3585 3460 53 59 50 3695 3560 3440 61 74 64 3705 3575 3460 at 2170 A. in isopropyl alcohol. '
E
F
G
Ha
3330 53 3330 52 3330 52 3335 51 3335 48 3330 52 3340 38 3335 42 3340 38 3335 46
3230 36 3230 34 3230 34 3235 34 3235 32 3230 34 3240 25 3240 27
3140 29 3140 24 3140 26 3145 24 3140 22 3140 26
2230 1410 2200 1440
..
.. ..
3140 21
.. .. .. ..
2210 1510
.. .. ..
.. .. ..
..
..
..
..
3235 30
3145 24
.. ..
..
..
..
..
2100
TABLE IV ABS~RPTION SPECTRA OF &BUTYL NITRITEI N VARIOUS SOLVENTS ULTRAVIOLET Wave lengths (A.) and molecular extinction coefficients (mole-1 1. cm.-l) Solvent
A
B
C
D
E
F
3405 3295 3970 3820 3660 3530 15 25 53 82 73 48 3405 3980 3820 3285 3660 3520 Isooctane 15 8 35 54 30 48 3405 3290 3970 3810 3660 3520 Carbon tetrachloride 12 23 47 71 62 41 3400 3970 3800 3285 3520 Chlorobenzene 3660 44 68 61 17 25 41 3400 3970 3810 3295 3530 Diethyl ether 3660 21 9 41 46 72 64 3295 Di-isopropyl ether 3400 3980 3810 3660 3530 21 46 71 64 11 40 3380 3730 .. 3480 p-Dioxane 3880 3600 24 37 52 .. 33 51 3395 3970 3800,3730 3275 3520 Ethyl acetateb 3660,3600 3880 3495 3400 3300 -4cetone 3730 3595 37 27 40 30 40 51 3070 3405 3295 &Butyl alcohol 3805 3660 3525 14 7 52 44 33 29 Water 3700 3840 3455 3575 .. .. a H peak at 2160 A. in methanol and at 2210 A. in isopropyl alcohol. Weak diffuse peaks. Cyclohexane
those observed for the n- and isoalkyl gitrites. The wave length maxima in the 3000-4000 A. range for t-butyl nitrite in cyclohexane, isoijctane and carbon tetrachloride appear in essentially the same positions as in the gas phase.8 However, in acetone and water, the wave length yaxima appear to shift downwards by 50 to 100 A. The wave length maxima for n- and isoalkyl nitrites in non-polar and slightly polar solvents are essentially the same as the peaks observed in the gas phase.8J1 The peak nzaxima appear t o shift slightly upwards in wave length in acetone and water. Actually, the wave length maxima for all the alkyl nitrites in acetone
G
Ha
..
2210 1680 2210 1710
..
.. ..
.. ..
..
.. ..
..
..
*.
.. ..
2210 1790
.. ..
.. .. .. ..
3165
..
..
.. .. ..
.. ..
..
..
2100
and water appear a t about the same positions. It should be noted that previous work" also shows a slight upward shift in peak maxima for n and isoalkyl nitrites when passing from the nitrites in gas state to the nitrites in noli-polar solvents and then into acetonitrile and dimethylformamide as solveilt s. The p-dioxane showed pecularities as a solvent for alkyl nitrites. The wave length maxima of the alkyl nitrites in p-dioxane closely approximated those in acetone despite wide differences in polarity. While the spectra of the n- and isoalkyl nitrites
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A. P. ALTSHULLER,I. COHENAND C. M. SCHWAB
in ethyl acetate were normal, tbutyl nitrite showed anomalous behavior. The wave length maxima were very weak, and a definite doubling of peaks occurred as if additional band system were appearing in this one instance. Possibly this result is due to transesterification forming some ethyl nitrite. All of the molar absorption coefficients (moles-' liter cm.-l) were less than 100 for members of the band system in the 30004000 A. region in all of the solvents used. The low intensities tempt one to associate these band systems with a forbidden n +.n transition. It has b:en suggested that the band maxima around 3500 A. found for the nitrite ion is the result of a similar transition in which one of the unshared electrons on the nitrogen atom is raised to an antibonding n orbital of the ion.'' I n alkyl nitrites, nitrous acid, dinitrogen tetroxide, dinitrogen tetroxide-solvent complexes,'* nitrosamines and @trite ion, a weak band is present around 3500 A. One might consequently associate an n + n electronic transition with all of these bands. There is some difference in these band systems in that the alkyl nitrites, nitrous acid, dinitrogen tetroxide-solvent complexes and nitrosamines show vibrational fine structure whereas dinitrogen tetroxide and nitrite ion show only a single broad band. Unfortunately, the data presented on the ultraviolet spectra of alkyl nitrites in various solvents here and elsewherel1J2 do not give convincing evidence for the blue shift which has been found to arise from n +- n type electronic transitions. l9 It has been pointed out previously in the literature'' that the intensity ratio in n and isoalkyl nitrites of the two peaks on either side of the most intense maxima at about 3560 A. varies with the solvent. That is, the D peak is more intense than the B peak in non-polar and slightly polar solvents, but the opposite is true in highly polar solvents such as acetonitrile and dimethy1forrnamide.l' Similarly, in this study, the same effect is noticeable going from non-polar to the polar solvents, acetone and water. Although the intensities of the nitrites in water were not obtained quantitatively, the B peak is more intense than the D peak. This variation in intensity ratio may be associated with the variation in the ratio of cis to trans rotational isomer with solvent. The rotational isomerism of alkyl nitrites is well established.20~8J0J1It has been shown in the gas state that the trans-form predominates (except for methyl nitrite), but that the ratio of trans to cis isomer increases in the order tertiary > secondary > primary nitrite.' It seems also reasonable that (17) W. G. Trawiok and W. H. Eberhardt, J . Chem. Phus., Z2, 1462
(1954). (18) A. P. Altshuller, THIEJOURNAL,62, GO7 (1958). (19) H. McConnell, J . Chem. Phus., 20, 700 (1952). (20) Dior and P. Tarte, ibid., 19, 10G4 (1951).
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a change in solvent from non-polar to highly polar might affect this ratio also. I n t-butyl nitrite, the B peak is more intense than the D peak, irrespective of solvent. If a ratio of B/D of greater than unity can be associated with a large preponderance of the trans-form for t-butyl nitrite in solution, as in the gas phase, then the nand isoalkyl nitrites would be shifted toward a larger proportion of the trans-form with increasing dielectric constant of the solvent. The lack of a blue shift in n- and isoalkyl nitrites for the 3000-4000 8. band system, if it actually does result from a n -+ n type electronic transition, may be associated with the shift in isomeric ratio acting in an opposite directiori t o the usual solvent effect in shifting the peak maxima. If the cis to trans ratio is less sensitive to solvent in t-alkyl nitrites, then the blue shift might be expected to show itself. Indeed, this is confirmed experimentally if the blue shift found for t-butyl nitrite in acetone and water is genuine. Relatively few data have been available on the short wave length peaks in alkyl nitrites. It has been reportedgml" that n-butyl- and n-amyl nitrite in a light petroleum mixture showed maxima a t 2220 and 2185 A., respectively, and that n-butyl nitrite has a maximum a t 2180 A. in ethanol. The present work indicates that theoalkyl nitrites show maxima in the 2210 t o 2240 A. region in hydrocarbons. It was also found that the n- andoisoamyl nitrites exhibit maxima at about 2220 A. in the gas state. When diethyl ether is used as, a solvent, the maxima are found a t about 2210 A. I n isopropyl alcohol, the alkyl nitrites show absorption maxima between 2170 and 2220 8.,while in methanol these compounds show peak maxima between 2110 and 2160 A. The alkyl nitrites absorption maxima between 2080 and 2100 A. in water. The slightly higher wave lengths for the peaks in hydrocarbons compared with the gas state may indicate a red shift owing to polarization.21 However, the more important characteristic of the data is the quite definite blue shift associated with the short wave length bands.1gs21 The absorption maxima in the 2050 to 2250 8. region have molecular absorption coefficients in the 1000 to 2000 mole-l liter cm.-l range. These intensities, along with the definite blue shift shown by this band, strongly suggest the assignment of the band as an allowed singlet-singlet n -+ n transition. Acknowledgment.-The authors wish to thank Donald Stephens for assistance in obtaining some of the ultraviolet spectra during the c-ourse of this investigation.
have
(21) N. S. Bayliss and E. G. McRae, THIEJOURNAL, 68, 1002, 1006 (1954).
