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Spectroscopic Analysis Using the Near-Infrared Region of the Electromagnetic Spectrum Elbert W. Crandall Pittsburg State University, Pinsburg, KS 66762 The near-infrared portion of the electromagnetic spectrum lies in the 0.8-2.5-pm region between the visible and middle-infrared. This region involves overtones due to anharmonic oscillators of the mid-IR in the 3-6-pm region and combinations of bending and stretching vibrations of the mid-IR. For example, the first overtone of the C-H stretching vibration a t 3.4-3.5-pm in the mid-IR shows up a t 1.7 pm in the near-IR for alkyl C-H and a t 1.6 pm for aryl C-H. Combination C-H bands appear in the 2.0-2.5-pm region. Carbonyls show the first overtone of the 5.7-pm hand a t 2.8 pm and the second overtone a t 1.9 pm. Compounds containing hydroxyl groups show the first overtone of the 2.8-pm 0H stretch a t 1.4 pm, while N-H shows overtones a t 1.4 pm for aryl N-H and 1.5 pm for alkyl N-H along with combination bands in the 1.9-2.0-pm region. Wheeler (1, 2 ) has written excellent reviews of the literature up to 1959, while Goddu and Delker ( 3 ) provide a table showing all of the overtone and combination hands of various C-H, N-H, OH and carbonyl compounds known to 1960. The near-IR has been used very little to study high polymers. Work a t Pittsburg State University has shown that this technique can he very useful to monitor high-polymer . . . systems. his paper is an &tempt to review the use of nearIR for simple compounds since the review by Wheeler and to review thiuse of near-IR in the area of polymers. Sample handling in the near-IR in some ways is simpler than for the mid-IR. Samples can he run neat in silica cells, as films between glass plates for polymers, and in chlorinated solvents as carbon tetrachloride, carbon disulfide, and chloroform. Amlnes A number of workers have studied the spectra of the N-H bond of amines. Whetsel, Roherson, and Krell (4, 5 ) have looked a t the near-IR spectra of aryl amines while Lohman and Norteman (6) have investigated primary, secondary, and tertiary alkyl amines. All of the amines show the first overtone of N-H stretching a t 1.45-1.52 pm with the aryl hand a t slightly shorter wavelength than the alkyl hand. Primary and secondary amines show this band; tertiary do not. Alkyl amines all show a combination band a t 2.023 pm while aryl amines have this band a t 1.97 pm. Whetsel, Roherson, and Krell (7) have used these combination bands to measure auantitativelv the concentration of aniline in the presence bf N-ethylaniline with a standard deviation of &0.1%.Stage, Stanley, and Moseley (8)haveused the 1.5-pm and 2.0-pm bands for primary amines and the 1.53-pm band for secondary amines as an analytical method for determining the concentration of amines in the presence of nitriles with average errors o f f 0.001% to f0.013%.
466
Journal of Chemical Education
Hydroxyl Compounds The 0-H stretching vibration that amears a t 2.8 um in the mid-IR shows theufirst overtone a t i:4 pm for alcohols, ~henols.and water. For alcohols and ohenols there is a weak combination hand a t 2.0 pm, while fbr water this combination band lies a t 1.9 um. Because of this, several methods have been developed to monitor water, 'alcohols, polyols, etc., in industrial procedures. For example, Jones and Brown (9) have used bands a t 1.5 and 2.1 pm bf polyols for the online monitoring of polyol-polyether manufacturing. For example, they measured water in the 0-1000-ppm range in polyols. Turley and Pietrantonio (10) have used the comhination bands of water and polyols to determine the hydroxyl number of polyols and the water content of polyols in the 0.01-0.3% ranee. Subramanian and Fisher (11) used the first overtone a t 1.46 pm to distinguish hetween'hokd water and bulk water in oroteins. Water H-bonded to an amine e r o w shows an absoiption a t 1.406 pm as opposed to hulk water at 1.46 um. Union Carbide (12) has develped a continuous process :idyic.r uaing the difference in the positions of the c~mihinationhands uf H,O and alruhols in thr 1.9-2.1-rrm region to monitor the of water in alcohols, and Long (13) reports a method to analyze for water in tobacco by using the 1.94-pm combination band of water. He was able to measure water in the 10-50% range with a standard deviation of 0.06% compared to an oven-drying method. Carbonyl Compounds The C=O of aldehydes, ketones, acids, and esters shows the first overtone of the 5.7-pm stretch a t 2.71 pm in the midIR and the second overtone a t 1.97 pm in the near-IR. The first overtone a t 2.71 pm appears as a strong hand in all of the above systems; however, the presence of the second overtone depends on the groups attached to the carhonyl. For example, in unpublished work that we have carried out in our laboratories (14) we have found that alkyl and aryl aldehydes, aliphatic ketones, and cyclic ketones show the second overtone. On the other hand, in aryl ketones this hand is absent. For example, benzophenone and 1-acetonapbthone show the first overtone a t 2.71 but no second overtone. In the case of aldehydes and ketones we have found the following order for the intensity of the carbonyl second overtone: H RC=O
0
I1
> R-C-R
> cyclic ketones > H Arc-0
0
I1
> Ar-C-CH3
0
II >> Arc-Ar
Holman and Edmonson (15) found that a series of acids
and esters they studied had strong absorption a t 2.72 and 2.83 pm due to the C=O first overtone and carboxyl OH. However the second overtone of C=O in the 1.90-um reeion was found to decrease in intensity with chain length. ~ G e r s all showed the 2.71-um first overtone. but lone chain acids and esters showed no 1.90-pm band. Powers, Harper, and Tai (16) have found that aromatic aldehydes show two strong bands a t 2.21 and 2.246 pm, which they assign to combination bands of the formyl C-H. We have found the same two bands for a series of aliphatic aldehydes (14). Acid anhydrides that show the carhonyl stretch as a doublet at 1790 and 1760 em-' in the mid-IR do not have the second overtone a t 1.97 um. This is an advantaee when looking a t epoxy systems'cross-linked hy acid a&drides. The curinn can be followed bv measurina' the increase of the 1.97-pm b k d due to ester fogmation. Unsaturalton The first overtone of the C-H stretching band a t 3.5pm in the mid-IR shows up at 1.7 pm for alkanes. When the hydrogen is attached to a trigonal carbon of a carbon-carbon double bond, the first overtone lies a t shorter wavelengths. Goddu (17) has found that the C-H of a terminal double bond gives the first overtone a t 1.62 pm and a combination band at 2.1 pm. Cis unsaturation shows bands at 1.62 and 2.14 pm. Polymers The near-IR has not been used to study polymers to the extent that other regions of the electromagnetic spectrum have. A few workers-have looked at step-reaction systems. Foster, Row, and Griskey (18) have observed the C-H overtones of vinyl and polyamide systems. Goddu and Delker (19) used the 1.65-pm and 2.20-pm bands of terminal epoxy C-H to studv euoxv resins. while the OH content of nolvmer systems ha& been"measuied by Hilton (20) using cbmbination hands a t 2.2-2.3 um and bv Miller and Willis (21) . . usine the 1.42-pm first overtone of the O-H group. We have studied several polymer systems in our laboratories (22-24). We have looked a t polyamic acids, epoxy, phenol-formaldehyde, polystyrene, polyesters, urea-formaldehyde, nylon 66, polyurethanes, and polyethylene. All of the polymer systems studied showed the first C-H overtone in the 1.67-1.70-fim region. This hand can be used as an internal standard to measure changes in other hands according to the method of Henniker (25). . . Combination bands of alkvl CH appear as three intense hands in the 2.3-2.5-pm region. Polymers with aryl rings show a doublet a t 2.13 and 2.16 pm, and a very intense band at 2.5 pm. These C-H combination hands vary from system to system (23) but remain constant within a given system during curing (24). The near-IR has proven to be a valuable technisue for following curing processes. For example, the imidizakon of
polyamic acids can be followed by looking a t the decrease of two bands. The 1.95-pm N-H hand of aryl amines and avery intense hand at 2.3 pm assigned to 0H
II I
-C-N-
both disappear on ring closure of polyamic acids going to oolvimide (22). E.~ o . x vsvstems " cross-linked bv uhthalic anhydride can be monitored (24) by following the disappearance of a band a t 2.19 um that has been shown to he a hydrogen attached to an kpoxy group (19). In addition during curing of the epoxy system, the O-H hand at 1.42pm and the ester C = O band a t 1.91 pm undergo increases. Kirkbright and Menon (26) have been able to measure the amount oi' vinyl acetate in a polyvinyl chloride-polyvinyl acetate copolymer by looking a t the second overtone of the ester C=O a t 2.15 um. while Buback (27) has heen able to observe the curing'of bolyethylene using the difference in the position of the first C-H overtones of ethene and ~ o l v Concluslon The near-IR region of the electromagnetic spectrum has proven to he a very valuable tool although i t bas not been used to the extent that the mid-IR has. The near-IR region consists of overtone and combination bands of C-H, N-H, O-H, and C=O. This region has been found to he very useful in the analysis and monitoring of reactions of amines, alcohols, phenols, acids, aldehydes, ketones, and polymer systems. Literature Clted
Volume 64
Number 5
May 1987
467
