11. L. A. Woodward, in Raman Spectroscopy, H. A. Szymanski, Ed. (Plenum Press, New York, 1967).

12. I. I. Kondilenko, V. E. Pogorelov, and K. Khue, Op. Speetrosc. 26, 518 (1969).

13. G. P. Buyan, I. I. Kondilenko, and V. E. Pogorelov, Opt. Spectrose. 27, 132 (1969).

14. F. C. Strong, Appl. Spectrosc. 23,593 (1969). 15. J. Loader, Basic Laser Raman Spectroscopy, (Heyden &

Son, London, 1970), p. 7.

16. Deviation from standard band shape usually appears in the band wings. A wing correction is usually applied to band parameter (e.g., peak height) to correct for discrepancies of intensity calculated with the assumption of standard curve shapes. While no wing corrections were used in our peak height calculations, the area measurements were taken of the actual band shapes as displayed on the curve analyzer. See, for example, Refs. 5 and 17.

17. A. S. Wexler, Appl. Spectrosc. Rev. 1, 29 (1967).

Far Infrared Group Frequencies. II. Primary Amines

Stephen M. Craven*

Department of Chemistry, Miami University, Oxford, Ohio 45056

Freeman F. Bentley

Air Force Materials Laboratory (LPA), Wright-Patterson A FB, Ohio 45433

(Received 13 December 1971; revision received 2 February 1972)

The far infrared spectra of a number of aliphatic amines have been recorded as dilute solutions in cyclohexane. A characteristic band with center at 225 ± 10 cm -1 is present in the spectra of aliphatic primary amines with a primary a-carbon atom. On the basis of the deuterium shift this band is assigned to the amino torsion. The effects of branching at the a-carbon, coupling of methyl and amino torsions, and intramolecular association on the frequencies of NH~ tor- sions are discussed. INDEX HEADINGS: Far infrared spectroscopy; Amines; Torsional vibrations.

INTRODUCTION

Previously we reported 1 the far infrared spectra of a number of aliphatic alcohols as dilute solutions in cyelo- hexane and pointed out that the hydroxyl torsion gives rise to a relatively intense characteristic band in the far infrared. The position of this band is useful for studying intermolecular and intramolecular association, and making spectra structure and spectra potential func- tion correlations. In this work we have extended our studies to amino group torsions of primary amines.

I. EXPERIMENTAL METHOD

All amines were obtained from Chem Service and used without purification. The ND2 compounds were prepared by adding a few drops of D20 to a dilute solu- tion of amine in cyclohexane and shaldng for a few minutes. 2 The cyclohexane layer was removed with a syringe and transferred to polypropylene far infrared cells. Samples of dilute solutions were recorded in 5-ram cells, whereas samples of the pure amines were recorded in cells less than 0.5 mm.

The far infrared spectra were recorded from 75 to 500 cm -~ with the Digilab model FTS-14 interferometer at

* Address correspondence or requests for reprints to Dr. Stephen M. Craven, Miami University, c/o AFML/LPA, Wright-Patterson Air Force Base, Ohio 45433.

the Mellon Institute, Carnegie-Mellon University. Res- olution varied from 8 to 4 cm -~, and 200 scans were averaged for any particular sample. The sample and background spectra were ratioed in order to remove background features. Spectra of amines were recorded as 1.0 to 0.1% by weight amine in cyclohexane in order to insure that predominantly monomer was present in solution.

II. DISCUSSION AND RESULTS

A. n-Propylamine

In Fig. 1 the infrared spectra from 425 to 75 cm -1 are presented for n-propylamine liquid and a 1.0 % solution of n-propylamine in cyclohexane. The broad feature in the liquid spectrum is the characteristic band which Stewart 3 reported at less than 290 cm -1 and assigned to the amino torsion influenced by intermolecular interac- tions, i.e., hydrogen bonding. The center of this band is estimated to be 260 cm -I in the spectrum of propyla- mine liquid and shifts to the 200 to 180 cm -1 region in the spectrum of liquid propylamine-N-d2. The large shift with deuteration confirms the torsional assignment of S tewar t / As the concentration of amine in cyclo- hexane is gradually decreased, the broad band present in the liquid becomes narrower and shifts to lower frequencies. The lower spectrum in Fig. 1 is that of a 1.0 % solution of propylamine in cyclohexane. At this

Volume 26, Number 4, 1972 APPLIED SPECTROSCOPY 449

Z O

a_ gg O

P I ~ t 4 0 0 300 2 0 0 I00

WAVENUMBERS CM -t

FIe. 1. Top, infrared spectrum of n-propylamine liquid, path length 0.2 mm; bottom, infrared spectrum of 1.0% solution of propylamine in cyclohexane, path length 5 ram.

high dilution, predominant ly monomer exists in solu- tion. In the spectrum of propylamine in eyclohexane, a relatively intense band is centered at about 220 cm-L This band is not present in the solution spectrum of propylamine-N-d~, but instead a strong band appears with center about 165 cm -~ (Fig. 2). The ratio ~/VD ~-~ 1.33 indicates tha t the bands centered at ~ 2 2 0 and ~ 1 6 5 cm -~ result f rom the NH2 and ND2 torsional vibrations, respectively. The absorption scales of both spectra in Fig. 1 have been expanded in order to facili- ta te the analysis of weaker features. One can see tha t there are a number of weak features on the low fre- quency and high frequency sides of the band centered a t about 200 cm -~ in the spectra of n-propylamine dis- solved in cyclohexane. Since n-propylamine can exist in five distinct conformations, each possessing a unique spectrum, a total of 25 low frequency bands due to fundamentals may appear in the spectrum. The five distinct conformers are denoted tt, tg, gt, gg, and gg'. The first t or g corresponds to dihedral angles between the plane C-C-C and C-C-N of 180 ° and 60 °, respectively,

I I I 500 400 300 200 I00

WAVENUMBERS (CM")

FIG. 2. Infrared spectrum of 1.0% solution of propylamine- N-d2 in cyclohexane, path length 5 ram.

TABLE I. Tentative assignment of the low frequency infrared spectrum of n-propylamine 0.5% in cyclohexane. ~

CH3CH2CH2NH2 CH3CH2CH2ND2 Assignment

328 325 C- -C- -C- -N def g 278 sh 270 C- -C- -C- -N d e f t

N250 sh 223-245 CH3 torsions t and g ~220 ~,170-165 NH2 torsions t and g 157,140 sh 157,138 sh Skeletal torsions t and

g

Abbreviations: sh, shoulder; g, gauche CCCN skeleton; t, trans CCCN skeleton.

and the second t, g, or g' denotes dihedral angles be- tween C-C-N and the plane containing C-N and the lone pair on nitrogen of 180 ° , 60 ° , and --60 ° . The five fundamental modes of vibrat ion of each conformer which are expected to have frequencies less than 500 cm - I are the two skeletal deformations, a skeletal tor- sion, a methyl torsion, and an amino torsion. The highest frequency skeletal deformations for each con- former are expected to have frequencies greater than 400 cm -1 similar to butane, the skeletal torsion will occur at less than 200 cm -1, and the methyl and amino torsions are expected to have frequencies less than 250 cm -1.

In Table I, the tenta t ive assignments for the low frequency bands of propylamine are presented. The shoulder at 278 cm -1 in the spectrum of the 1.0 % solu- tion of n-propylamine in cyclohexane appears as a ra ther sharp band in the spectrum of the ND2 com- pound at 270 cm -1 (Fig. 2). The 278 cm -1 is assigned as the lowest frequency in-plane C C C N deformation of the t rans conformer, whereas the 328 cm -~ band must result f rom the conformer with the gauche skeletal arrangement. The lower frequency band is assigned to the skeletal deformation of trans conformer because the effects of deformation-deformation interaction and skeletal s tretching-deformation interaction are expected to be greatest for the conformer with an azimuthal angle of 180 ° for the C C C N skeleton. 4 Confirmation of this assignment will have to await a s tudy of the tem- perature dependence of the low frequency spectrum of

450 Volume 26, Number 4, 1972

TABLE II. NH~ and ND~ torsional frequencies.

Compound NH~ ND~ A m i n e i n

cyc lohexane (%)

n-Propylamine 220 165 n-Butylamine 220 165 n-Hexylamine 225 - • • n-Hexydecylamine 226 - • -

1,6-Hexanediamine 225 • • •

Isobutylamine ~220 ~165 Isopropylamine ~230 - • • Secondary butylamine --~220 162 Cyclohexylamine 218 165 Tertiary butylamine 256 177 3-Hydroxypropyla- 250 • • •

mi ne

3 - M e t h o x y l p r o p y l a - 246 • - - 0.1 mine

3,3-Diaminopropyla- 240, 220 .-. 0.2 mine

1,1-Dimethylhydra- 280 • • • 0.5 zine

0.5, 0.5 0.3, 0.3 0.5 Saturated

solution 0.5 0.3, 0.3 0.5 1.0, 1.0 0.2, 0.2 0.3, 0.3 1.0

CH3CH2CH2ND2. The weaker features in the 245 to 200 em -~ region result f rom at least two methyl torsions (t) and (g) and two amino torsions (t) and (g) and perhaps as many as five methyl and amino-torsional fundamentals and accompanying hot bands. Study of the n-propylamine-N-d2 spectrum indicates tha t for pr imary amines, the features associated with the NH2 and ND2 torsions are generally stronger than absorption bands due to the methyl torsional transitions. The effect of the skeletal deformations, skeletal torsions, and methyl torsions is to make it diificult to determine the band centers of the NH2 and ND2 torsional vibrations. In Table I I the band centers measured for some other long chain pr imary amines are tabulated. As the chain length increases, distortion of the NH2 torsional band as a result of contributions from the skeletal deforma- tions decreases. In some cases there is a symmet ry and structure on the bands resulting from the NH~ torsional transitions, but the spectra of long chain amines in dilute cyclohexane solutions can be said to have a characteristic band with center at 225 ± 10 cm-L These bands are broad; for example, the width at half-height of the NH2 torsional band in the spectrum of 0.5% solution of n-hexylamine in cyclohexane is about 130 cm-L I t is of interest to note tha t Scott and Crowder ~ assigned the NH~ and CH~ torsions to Q-branches in the spectrum of propylamine at 210 and 252 cm -~, re- spectively, but could find no clue as to which of the 5 rotomers of n-propylamine predominates.

B. Tertiary Butylamine

Scott and Crowder ~ as well as Tsuboi et al . ~ have studied the far infrared spectrum of ter t iary butylamine vapor. The former researchers also studied the spectrum of (CH~)~CNH~ in alkane solvents. Recently, Carlson, Fateley, and Bent ley ~ reported the spectra of ter t iary butylamine and ter t iary butylamine-N-d~ but did not discuss the spectra.

The methyl torsional modes belong to the a ' and 2a"

,oo I I i r

g 80 , ---~ , -

~ 6 c - I

~- 4 c -~ . . . .

o . l . I I ! 4 0 0 5 0 0 2 0 0 1 0 0 5 0

W e v e n u m b e r i n c m "

Digi lob FTS-14 Sample tert-Butylam|ne-NDz 0.3% Cyclohexane Source Chem" Service Cell 5 Mi l l i~ ter Resolution B Scans 200 Structure: (CH3)3CND2

FiG. 3. Infrared spectrum of 0.3% solution of tertiary butyl- amine-N-d2 in cyclohexane, path length 5 ram.

symmet ry species whereas the amino torsion belongs to the a" species, assuming (CH3)~CNH2 has Cs sym- met ry similar to methylamine. Scott and Crowder ~ as- signed the NH~ torsion to 253 cm -1 Q-branch, and, since the a" methyl torsions would be expected to gain some intensi ty as a result of coupling with a" amino torsion, they were assigned to the 200 and 273 cm -~ Q-branches. Tsuboi 6 preferred to assign the 245 cm -~ Q-branch to the amino torsion. In the spectrum of (CH3)~CNH2 dissolved in cyclohexane, 2 a strong band appears at 256 cm-1; however, if this band and the one at 177 cm -1 in the spectrum of (CH~)3CND2 (Fig. 3) are assigned to the NH2 and ND~ torsions, respectively, the ratio ~/~D = 1.44. This value is greater than the 1.41 expected for a harmonic vibrat ion and would seem to indicate tha t the NH2 fundamental is involved in Fermi resonance or perhaps couples with a lower frequency a" methyl torsion. Because of the apparent per turba- tion of the true NH2 torsional frequency, we compare the 177 cm -~ ND2 torsional frequency with the ~--465 c m - ' value for ND~ torsions of long chain amines. Not only is the ND2 torsion at a higher frequency for (CH3)3CND2 as compared to CHaCH2CH2ND2, but the band width is considerably less. This is expected since ter t iary butylamine can exist in only a single conforma- tion; i.e., the barrier to amino group rotat ion is ex- pected to be threefold symmetric.

C. Pr imary A m i n e s with Secondary a-Carbon A t o m s

The simplest amine with a secondary a-carbon a tom is isopropylamine. Scott and Crowder ~ have examined the spectra of the vapor and of a dilute solution in eyclohexane. The amino torsion was assigned to 232 cm -1 Q-branch and the two methyl torsions were as- signed to Q-branches a t 201 and 263 cm -1. I t was concluded tha t the predominant conformation must be the one with C1 point group symmetry , with all three torsions of the same symmet ry species, ra ther than the one of Cs point group symmetry , with one methyl torsion of species a ' and the amine and other torsion of species a". These conclusions should be reconsidered since the two methyl and amino torsions are predicted to be active by the infrared selection rules for both the C1 and C~ conformers.

Tsuboi ~ has also analyzed the spectrum of the vapor and found evidence for t rans (Cs) and gauche (C1) con-

APPLIED SPECTROSCOPY 451

z L/ o_

I ] I 500 400 300 200 I00

WAVENUMBERS (CM -~)

FzG. 4. Infrared spectrum of 0.2% solution of cyclohexyl- amine, path length 5 ram.

formers. Evidence for the presence of two conformers of (CH3)2CDNH2 was found from the analysis of the C-D stretching bands in the spectrum of a sample in dilute CC14 solutionJ The enthalpy difference was determined to be 0.12 ~ 0.02 kcal/mole with the trans conformer (C, point group) being the most stable. Similar results were found for other molecules, CH3CD2NH2 and CD~CH2NH2. Assuming that the conformational equi- librium does not differ much for alkane solvents, one can conclude tha t the band centered at 233 cm -~ in the spectrum of (CH3)2CHNH2 in alkane solvent reported previously results from superposition of the six torsional fundamentals, the amino and two methyl torsional fundamentals for each conformer.

Since we could not accurately determine the fre- quencies for the amino torsions of isopropylamine, the spectrum of secondary butylamine was recorded. In CH3(NH2)CHCH2CH3, only one methyl group is at- tached to the a-carbon. A relatively intense band is present in the spectrum of this molecule at 225 cm -1. The band is fairly broad, as expected for a molecule which exists in a number of different conformations. The ND2 torsion occurs at 164 cm -~ in the spectrum of 1.0 % solution of secondary butylamine-N-d2 with cyclohexane.

In order to obtain the NH2 torsional frequency for a compound which contains no methyl groups attached to the a-carbon, the spectra of cyclohexylamine and the corresponding ND2 compound were recorded (Figs. 4 and 5) as dilute solutions in cyclohexane. Cyclohex- ylamine may exist in four spectroscopically distinct conformations. The NH2 group can occupy the axial or equatorial positions of the; chair form of cyclohexane, and for each of the conformations of the molecular skeleton the electron pair may be pointed toward or away from the center of the ring. Brignell, Brown, and Katr i t sky s have tried to determine the populations in these different states by an indirect method. Their re- sults indicate tha t the lowest energy form has an equa- torial NH2 group with the lone pair pointing toward the center of the ring [C~ point group].

The NH2 torsion is immediately identified in the spectrum of cyclohexylamine (Fig. 4) as the strong a b -

452 Volume 26, Number 4, 1972

[ E 500 400 300 200 I00

WAVENUMBERS (CM "l)

FiG. 5. Infrared spectrum of 0.2% solution of cyclohexyl- amine-N-d2, path length 5 mm.

sorption at 218 cm -1 (ND2 165 cm-1). The ratio VH/~D is 1.33. The band at 242 cm -1 does not appear to have a corresponding band in the spectrum of the ND2 com- pound. A ring vibration is expected in this region, and the 242 cm -1 band may result from it or perhaps the NH2 torsion of a second conformer. Both the NH2 tor- sion and ring vibration expected at ~ 2 5 0 cm -1 belong to the same symmetry species for the inner equatorial conformer. A slightly greater shift on deuteration for the 242 cm -1 band than that for the 218 cm -1 band would result in the merging of the two bands in the spectrum of the ND2 compound. The bands at 338,450, and 464 appear to be mass-dependent since they shift to 315, 443, and 455 cm -1 in the spectrum of ND~ com-

N I

pound. They must involve motion about C - - C H - - C part of the ring. In the spectrum of the ND2 compound, bands are present at 475 and 337 cm -1 and may result from skeletal deformations of some residual undeuter- ated cyclohexylamine. However, we prefer to at t r ibute these bands to the two ring vibrations which involve

C

little motion of N/CH--NH2 portion of the molecule. %

/

C This interpretat ion is consistent with the assignments and normal coordinate calculations for the somewhat analogous molecule, methylcyclohexane2

D . I n t r a m o l e c u l a r A s s o c i a t i o n

Intramolecular association in amino alcohols has been well established. The most stable form of amino alcohols is expected to be one in which the amino group is the proton aceeptor OH -.- iN. In order to determine the effect of this type of intramolecular association on the far infrared spectra, we tried to record the spectrum of NH2CH2CH2OH as a dilute solution in cyclohexane. Because of the limited solubility of this compound in cyclohexane, we were not able to obtain a spectrum. It has been reported in the literature I°, 11 that the series of amino alcohols (C2Hs)2N(CH2)~OH (n = 2, 3, 4, 5) are intramolecularly hydrogen-bonded and that the strong-

est intramolecular hydrogen bond is formed in a seven- membered ring, n = 4. With these results in mind, the spectra of 3-amino-l-propanol and 3-methoxypropyla- mine were recorded as dilute solutions in cyclohexane.

Comparing the spectrum of 3-methoxypropylamine with that for 3-amino-l-propanol, one notices that the usual broad, relatively intense band at 225 ± 10 cm -~ expected for amino torsions is present in the spectrum of the methoxy compound, but not the hydroxy. However, the band due to the amino torsion peaks at 246 cm -~ in the spectrum of CH3OCH2CH2CH2NH2. We attribute this to interaction of the hydrogens on nitrogen and lone pair electrons on oxygen. We have studied a num- ber of compounds in which the NH2 group may interact with phenyl, vinyl, and other amino groups, and both the position and intensity of the amino torsion bands are altered. We hope to report these studies at a later date.

III. CONCLUSIONS

This study has demonstrated that the amino torsion can be expected to give rise to the most intense band in the low frequency spectrum of a primary amine and therefore should be useful for group frequency correla- tions. Solution studies in cyclohexane using the long path cell and the deuteration technique developed by Carlson et al. 2 permit the easy identification of the torsional bands. Our results indicate that the NH2 tor- sion in long chain amines will occur at 225 ± 10 cm -1 for dilute solutions in cyclohexane. In the pure liquid the band will shift by 50 cm -1 for the short chain amines like n-propylamine and considerably less as the

amino group is diluted (in an intramolecular sense) by large hydrocarbon groups.

Studies of primary amines with branching at the a-carbon indicate that a simple correlation of the tor- sional frequency with alkyl substitution at the a-carbon is not very informative.

A C K N O W L E D G M E N T S

The authors thank Professor W. G. Fateley for his helpful suggestions and Mr. Frank Kurka of his labora- tory for preparing the ND2 compounds and recording the interferometric spectra. In addition, we thank Dr. J. E. Katon for the samples of n-propylamine-N-d2 and secondary butylamine-N-d2. This research was sup- ported in part by the United States Air Force Contract F33615-70-C-1021 with Miami University, Oxford, Ohio 45056.

1. S. M. Craven and F. F. Bentley, Appl. Spectrosc. in press. 2. G. L. Carlson, W. G. Fateley, and F. F. Bentley,

Spectrochim. Acta in press. 3. J. E. Stewart, J. Chem. Phys. 30, 1259 (1959). 4. T. Fujiyama, Bull. Chem. Soc. Japan 44, 1194 (1971). 5. D. W. Scott and G. A. Crowder, J. Mol. Spectrosc. 25,

477 (1968). 6. M. Tsuboi, A. Y. Hirakawa, and K. Tamagake, Nippon

Kagaku Zasshi 89,821 (1968). 7. P. J. Krueger and J. Jan, Can. J. Chem. 48, 3229 (1970). 8. P. J. Brignell, K. Brown, and A. R. Katritzky, J. Chem.

Soc. B p. 4345 (1968). 9. I~. G. Snyder and J. H. Schactschneider, Spectrochim.

Acta 21, 169 (1965). 10. N. Mori, E. Nakamura, and Y. Tsuzuki, Bull. Chem. Soc.

Japan 40, 2191 (1967). 11. V. P. Gardaenko, I. M. Ginzburg, and D. V. Cloffe, Opt.

Spektrosk. 27,337 (1969).

A Sensitive Emission Spectrographic Method for the Analysis of High Purity Mercury

H. R. Pahl

Chemical Products Plant, General Electric Lamp Division, Cleveland, Ohio 44110

(Received 20 December 1971; revision received 10 February 1972)

A method is presented for the analysis of high purity mercury. In comparison with methods for determining impurities in other matrices, this method is unique, both for its simplicity and for its low limits of detection. The sample is converted to HgO and burned in adc arc. The sample spectra are recorded on a photographic plate and impurities are determined by visual reference to spectra of standards photographed on the same plate. Detection limits are low enough to make the method useful for acceptance or rejection of 99.9999% purity mercury. This method is also useful for evaluating the purification of solutions of other metals by mer- cury pool electrolysis. INDEX HEADINGS: Mercury analysis; Trace analysis; Emission spectroscopy; Analytical methods.

INTRODUCTION Mercury was one of the earliest metals to be available

in high purity form, and 99.9999 % mercury has been in

commercial use for many years. It is fortunate that

relatively simple purification procedures are available

to provide mercury for critical applications. Fluorescent

Volume 26, Number 4, 1972 APPLIED SPECTROSCOPY 453