Infrared Spectra of Some a, -Dinitroxy Alkanes

R. D. Barefoot and A. R. Lawrence

U. S. Naval Propellant Plant, Indian Head, Maryland 20640

T h e i n f r a r e d s p e c t r a o f e i g h t a ,w-d in i t roxy a l k a n e s a r e e x a m i n e d , a n d f r e q u e n c y s h i f t s a n d r e l a t e d i n t e n s i t i e s of c h a r a c t e r i s t i c a b s o r p t i o n b a n d s a re d i scussed .

INTRODUCTION

In f r a red spectral data, like most physical measure- ments, do not provide a direct measurement of struc- ture, but they do provide certain parameters. In a series of related compounds, these observed param- eters should vary , with few exceptions, in regular fashion. This investigation was undertaken in hope of recognizing such correlations. The s tudy has been arbi t rar i ly limited to ~,~o-dinitroxy alkanes up to, and including, 1,10-dinitroxy deeane (excluding 1,8- dini t roxy octane).

Fu r the r work employing longer-chain carbon mole- cules and other oxidized nitrogen compounds is con- templated for the near future.

I. EXPERIMENTAL

The compounds were synthesized in these labora- tories by ni t ra t ing the appropriate alkane diols, and the purit ies of the ni t ra te esters were determined by vapor-phase chromatography. All solutions were pre- pared at 23.5°±1°C.

A Perk in -E lmer model 421 double-beam grating spectrophotometer was used and was calibrated with water-vapor spectra. Frequencies are believed accu- rate to ± 1 cm -~ throughout the region between 4000 and 550 cm -1.

1Vfeasurements were made in carbon tctrachloride solutions, and solute concentrations were maintained at 0.5 mole/ l i ter throughout. The temperature varia- tion during the measurements was ± I ° C . All band

positions are expressed in reciprocal centimeters (era-l).

140

~u

ii0 '.~

.~ 90

N 70

g 2 .5o

30

• 5 6 7 8 9 10 Number of Methylene Groups

FIG. 2. ep f o r 2950-em -~ b a n d of C t I i n CCI~ so lu t ion .

II. RESULTS

The infrared spectra of covalent nitrates have been studied by a number of investigators. 1-~ In each in- stance the two strong inf rared bands near 1640 and 1280 cm -1 have been identified with the asymmetric and symmetric stretching frequencies of the NO2 group. In addition to this, a strong band near 860 cm -1 and weaker ones at 760 and 700 em -1 have been

~u

40 •

~ ao

~ I I I I I I t I I I 2 3 4 5 6 7 8 9 10

Number of Methylene G r o u p s

FIG, I. e~ for 1465-cm -x band of CH in CCI~ solution.

380 Volume 20, Number 6, 1966

assigned. The 860-cm -~ band is thought to be due to an ON02 stretching vibration, and the one at 760 cm -~ due to NOe out-of-plane rocking. ~, ~, * Although there is less cer ta in ty about the 700-cm -~ band, i t has been assigned ~,~ as an overtone o£ the 351-cm -~ C - O - N bending vibration.

Similarly, the n-alkanes have been extensively studied and vibrat ional assignments made2 Rose v measured molecular extinction coefficients of hydro- carbons in the 5400-8900-em -~ region and observed that, for n-alkanes, t h e molecular extinction coef-

fieient E increased with CH2 addition. Similar meas- urements have also been made by l~IcMurray a n d

Thornton 8 and by Jones. 9 Examination of the methylene scissoring region

located between 1470 and 1425 cm -1 indicates that the absorption intensities of ~-alkanes are linearly related to the number of C - I t groups present in these molecules. ° There was good reason l° to believe that a similar circumstance might obtain for a,~- disubstituted alkanes and in part icular for a,~-di- ni t roxy alkanes.

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FREQUENCY (Ct~l

F z e . 3. S t f i f t of CH2 stretching frequency from 2965 e m -1 i n ( a ) E G D N to lower f r e q u e n M e s dlnitroxyhexane and (e) 1,10-dinltroxypentane.

! . . . . . . . . . . . ~ , 2 0 . . . ~

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a s s e e n i n ( b ) 1 ,6-

APPLIED SPECTROSCOPY 381

2900

2890 •

% 2880

2870

2860

2850

I I I I I i I i 2 3 4 5 6 7 8 9 10

Numbez ~ of Methylene G r o u p s

l~m. 4. Absorption band shift of a CH~ stretching frequency from 9894 em -1 in EGDN to 2852 cm -1 in 1,10-dinitroxypentane.

This belief was borne out by the measurements, and the results are shown in Fig. 1, which is a plot of apparent molecular extinction coefficient against the number of methylene groups in the molecule. Least- squares treatment of these data yields a slope of 4 and an intercept of 1, which is in only fair agreement with the work of Jones 1°, who found a slope of 8 and intercept of 35 for long-chain methyl esters.

A similar circumstance holds in the asymmetrical methylene stretching-frequency region between 2970 and 2925 em -1. Once again a plot of apparent molec- ular extinction coefficient against number of meth- ylene groups (Fig. 2) yields a line with a slope of 21 and an intercept of -75 , which compares poorly with Jones '1° plot (slope +75, intercept - 5 0 ) for long- chain methyl esters in the same frequency region. ~Here the values for the two-, three-, and four-carbon compounds have not been used since, as might be expected, the strongly polar nitroxy groups seem to produce considerable distortion.

The poor agreement of these two slope values with the results of Jones may have occurred because of two differences in experimental procedure. First, our

measurements were made with neat compounds while those of Jones were made with carbon tetrachloride solutions. Therefore, differences could have arisen due to concentration effects. Secondly, a different range of compounds was used in this study (four-10 methylene groups versus four-34 methylene groups), and this may have caused a difference in the apparent intensity per methylene group, although this ex- planation does not seem very likely.

Finally, it may be pointed out that the differences in the intercept values are to be expected since, for Jones' methyl esters, there is overlap of the methylene absorption with absorption bands of the terminal methyl groups.

Zbinden 11 suggested, in his discussion of vibrational interactions in chain molecules, that for some com- pounds the position of an absorption band may shift toward lower frequencies with increasing chain length. Observations made during this study support this suggestion. For example, the band at 2965 cm -1 in ethylene glycol dinitrate (EGDN), which was as- signed to the CH2 stretching frequency, shifted to lower frequencies with increasing chain length (see Fig. 3).

1640

1630

1625

1620

[

i I I I I I I I 3 4 5 6 7 8 9 10

N u m b e r of Methylene G r o u p s

FIa. 5. Shift of the NO2 band with increasing chain length.

382 Volume 20, Number 6, 1966

Similarly, the absorption band at 2894 cm -z in EGDN, which was assigned to a Ctt2 stretching fre- quency, shifted to lower frequencies with increasing chain length, as shown in Fig. 4.

Although, as might be expected, there is no sig- nificant change in the intensi ty of the N 0 : absorp- tion bands, in the NO2 asymmetr ical s tretching band region of the spectrum (1626-1653 cm-1), the NO2 absorption band shifts to lower frequencies as the car- ben chain is lengthened. For example, the NO2 band, located at 637 em -1, in 1,3-dinitroxy pentane shifts toward lower frequencies such tha t the N02 band is located at 1635 em -1 in 1,4-dinitroxy butane, 1627 cm -~ in 1,7-dinitroxy heptane, and 1622 cm -~ in 1,10- dini t roxy decane (shown in Fig. 5).

As a result of this study, it is believed tha t the observed absorption band shifts to lower frequencies and the increase in vibrat ional intensity, result ing f rom increasing the number of CH2 groups in the molecule, can be of considerable diagnostic value to

analysLs or spectroscopists working with s t ra ight

chain a, co-dinitroxy alkanes.

ACKNOWLEDGMENT

This work was suppor ted by the Foundat ional Re- search P rog ram of the Bureau of Naval Weapons.

.~eeeived 6 January 2964

1. J,. C. D. Brand and T. M. Cawthon, J. Am. Chem. Soe. 77, 319.-323 (1955).

2 . J. ~. Brown, J. Am. Chem. Soc. 77, 6341-6351 (1955). 3. R. D. Guthrie and H. Spedding, J. Chem. Soe. 1960, 953-

959 (1960). 4. S. Krimm and C. Y. Liang, J,. Appl. Phys. 29, 1407-1411

(1958). 5. G. Rossmy, Chem. Ber. 88, 1969-1974 (1955). 6. L. J. Bellamy, The Infrared Spectra of Complex Mole-

cules (John Wiley & Sons, Inc., New York, 1958) 2nd. ed. 7. F. W. Rose, Jr., J. Res. Natl. Bur. Std. 20, 129 (1938). 8. H. L. MelV£urray and V. Thronton, Anal. Chem. 24, 318-

334 (1952). 9. R. /~. Jones, Speetroehlm Aeta. 9, 235-251 (1957).

10. R. N. Jones, Can. ft. Chem. 40, 321-333 (1962). 11. R. Zbinden, J. ~ol. Spectry. 3, 654-663 (1959).

APPLIED SPECTROSCOPY 383