Further observations on the use of indene for the wavenumber calibration of infrared spectrometers

SpectrochimicaActa, 1964,Vol.20, pp. 1175to 1183. PergamonPress Ltd. Printedin NorthernIreland

Further observations on the use of indene for the wavenumber calibration of infrared spectrometers* R. N O R M A N JONES and A. N A D E A U Division of Pure Chemistry, National Research Council of Canada, Ottawa, Canada (Received 14 October 1963) Abstract4-New values are reported for the positions of the absorption maxima in the infrared spectrum of indene containing 0.8% by weight of both camphor and cyclohexanone. The personal error in locating the maxima has been reduced by fitting polynomial series of the second, third and fourth degrees through five points in the neighborhood of the maxima, and computing the roots of the first derivatives, using a computer technique previously described. The peak values are compared with independent measurements made by P. J. Krueger on a different spectrometer. The peaks most suitable for calibration purposes have been selected on the basis of their distribution in the spectrum and the low r.m.s, variation in the computed positions of the maxima. INTRODUCTION LIQUID indene c o n t a i n i n g small a m o u n t s of c a m p h o r a n d c y c l o h e x a n o n e was proposed in 1958 as a c o n v e n i e n t m a t e r i a l for t h e w a v e n u m b e r c a l i b r a t i o n of i n f r a r e d s p e c t r o m e t e r s of low a n d m e d i u m resolution [1]. I n 1961 t h e positions of 79 b a n d s b e t w e e n 3930 a n d 690 cm -1 were r e p o r t e d for a solution c o n t a i n i n g 98-4% o f indene (by weight) a n d 0.8 % each of c y c l o h e x a n o n e a n d c a m p h o r , as m e a s u r e d on a n i t r o g e n p u r g e d high-resolution single-beam g r a t i n g s p e c t r o m e t e r [2]. These d a t a were s u b s e q u e n t l y i n c l u d e d as p r o v i s i o n a l values in t h e " T a b l e s of W a v e n u m b e r s for t h e Calibration of I n f r a r e d S p e c t r o m e t e r s " p r e p a r e d b y t h e Commission on Molecular S t r u c t u r e a n d S p e c t r o s c o p y of I U P A C [3]. The s p e c t r u m of t h e same b a t c h of indene solution was s u b s e q u e n t l y m e a s u r e d i n d e p e n d e n t l y b y P. J. K r u e g e r , using a modified B e c k m a n I . R . 7 s p e c t r o p h o t o m eter w h i c h h a d been c a l i b r a t e d w i t h t h e v a p o r p h a s e c a l i b r a t i o n d a t a f r o m t h e I U P A C W a v e n u m b e r Tables [4]. The t w o series o f m e a s u r e m e n t s are c o m p a r e d in t h e left h a n d c o l u m n s of Table 1. F o r s i x t y - f o u r of t h e bands, t h e a g r e e m e n t is w i t h i n t h e tolerances set b y JONES a n d NADEAU [2], a n d o f t h e fifteen b a n d s where larger differences occur, nine h a d been d e s i g n a t e d as less suitable for a c c u r a t e c a l i b r a t i o n purposes, because of s u p e r p o s i t i o n on w a t e r v a p o r bands, gross a s y m m e t r y , or for o t h e r reasons. The r e m a i n i n g b a n d s for w h i c h significant discrepancies o c c u r are No. 2, 3, 34, 37, 66 a n d 67.

* Published as Contribution No. 7954 from the Laboratories of the National Research Council of Canada. [1] R. N. JONES, P. K. FAUREand W. ZAHARIAS,V I I ~hColloquium Spectroscopicum Internationale, Li@ge, Belgium 1958; Rev. Universelle mines 15, 417 (1959). [2] R. N. JONES, N. B. W. JONATHAN,1VLA. MACKENZIEand A. NADEAU, Spectrochim. Acta 17, 77 (1961). [3] Tables of Wavenumbers for the Calibration of Infrared Spectrometers. Prepared by the Commission on Molecular Structure and Spectroscopy of the International Union of Pure and Applied Chemistry. Reprinted from Pure and Applied Chemistry 1, No. 4 (1961) Butterworths, London (1961). [4] P. J. KRUEGER, Applied Optics 1, 443 (1961). 1175

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R. NORMAN JONES and A. NADEAU

I n all o f these i n v e s t i g a t i o n s t h e position o f t h e m a x i m u m was e s t i m a t e d visually, a n d appreciable personal e r r o r m a y occur, p a r t i c u l a r l y if the b a n d is significantly a s y m m e t r i c . * W h e r e t h e m e a s u r e m e n t s are m a d e on a high-resolution single-beam s p e c t r o m e t e r , n a r r o w " s p i k e s " due to residual w a t e r v a p o r a b s o r p t i o n can influence t h e s u b j e c t i v e j u d g e m e n t in locating t h e position o f t h e m a x i m u m of t h e u n d e r l y i n g envelope. A l t h o u g h d o u b l e - b e a m i n s t r u m e n t s are n o t subject to this defect, care m u s t be t a k e n t h a t t h e y are k e p t well p u r g e d w i t h d r y air or n i t r o g e n or t h e b a n d profile m a y be d i s t o r t e d because of e n e r g y deficiency w h e n traversing the water vapor band. T o r e d u c e t h e s u b j e c t i v e errors in t h e d e t e r m i n a t i o n o f t h e positions o f b a n d m a x i m a , a t e c h n i q u e has been d e v e l o p e d for c o m p u t i n g t h e position of the m a x i m u m a n a l y t i c a l l y [5-7], a n d this has n o w been applied to s i x t y - t h r e e bands in t h e s p e c t r u m of t h e i n d e n e - c y c l o h e x a n o n e - c a m p h o r s y s t e m . These d a t a p r o v i d e m o r e a c c e p t a b l e calibration s t a n d a r d s a n d define more precisely t h e residual uncertainty of the measurements. EXPERIMENTAL METHODS AND RESULTS T h e s p e c t r a were m e a s u r e d on a n i t r o g e n p u r g e d P e r k i n - E l m e r l 1 2 G doublepass g r a t i n g s p e c t r o m e t e r u n d e r t h e same conditions as were r e p o r t e d p r e v i o u s l y [2]. E a c h s p e c t r u m was s c a n n e d a t least twice o v e r t h e region o f t h e b a n d maxim u m , a n d t h e d a t a f r o m each c u r v e were a n a l y s e d s e p a r a t e l y . T h e position of t h e m a x i m u m was first e s t i m a t e d visually, two a d d i t i o n a l o r d i n a t e values were m e a s u r e d on e i t h e r side o f t h e p e a k w i t h i n one h a l f o f t h e half b a n d w i d t h f r o m t h e center. T h e s e d a t a were processed on a n I.B.M. 1620 c o m p u t e r b y P a r t I o f t h e b a n d profile analysis p r o g r a m [6, 7]. This p r o g r a m fits p o l y n o m i a l e q u a t i o n s o f t h e second, t h i r d a n d f o u r t h degrees t h r o u g h the m e a s u r e d points, differentiates t h e r e s u l t a n t f u n c t i o n s a n d p r i n t s o u t t h e roots t h a t yield s t a t i o n a r y o r d i n a t e values o f t h e first derivative. T h e significant roots for t h e cubic a n d quartic f u n c t i o n s are selected b y inspection, a n d t h e c o r r e c t e d positions of t h e b a n d m a x i m a are o b t a i n e d b y a d d i n g t h e abscissal co-ordinates algebraically to the e s t i m a t e d p e a k position. T h e p r o g r a m t h u s gives t h r e e values for each peak. T h e d u p l i c a t e sets of m e a s u r e m e n t s p r o v i d e six values for t h e p e a k position which were a v e r a g e d t o give a " r e c o m m e n d e d " value; a m e a s u r e of t h e a c c u r a c y of t h e fit is o b t a i n e d b y e v a l u a t i n g t h e r.m.s, v a r i a t i o n . F o r some b a n d s additional sets o f m e a s u r e m e n t s were also made. T h e results are s u m m a r i z e d in t h e c e n t r a l columns o f T a b l e 1. T h e " r e c o m m e n d e d v a l u e " a n d t h e r.m.s, v a r i a t i o n are r e c o r d e d in t h e r i g h t - h a n d columns. * The "visual estimation" of narrow prominent bands, measured at low noise levels, can be done by inspection. Where the noise level is significant, or the peak broad and asymmetric, our method is to pencil a smooth curve through the chart record, read a series of ordinate values in the neighborhood of the peak, and "home in" to the maximum. [5] R. N. JONES, K. S. SESHADRIand J. W. HOPKINS, Can. J. Chem. 40, 334 (1962). [6] R. N. JONES, K. S. SESHADRI, N. B. W. JONATHANand J. W. HOPKINS, Can. J. Chem. 41, 750 (1963). [7] R. l~. JONES, K. S. SESHADRIand J. W. HOPKINS, National Research Council of Canada, Bulletin No. 9 (1962).

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:DISCUSSION C o m p a r i s o n o f t h e n e w m e a s u r e m e n t s w i t h t h o s e we r e p o r t e d p r e v i o u s l y i n d i c a t e t h a t t h e d i s c r e p a n c y w i t h K r u e g e r ' s m e a s u r e m e n t s is e l i m i n a t e d for b a n d s No. 2 a n d 3 a t t h e high w a v e n u m b e r e n d of t h e s p e c t r u m ; t h e difference for B a n d 37 is also r e d u c e d , b u t still r e m a i n s significant, while t h a t for B a n d s 34, 66 a n d 67 is n o t i m p r o v e d . W h e n t h e s e n e w v a l u e s are c o m p a r e d w i t h t h e v a l u e s r e p o r t e d p r e v i o u s l y f r o m o u r l a b o r a t o r y , significant c h a n g e s are also o b s e r v e d for B a n d s 30, 41, 43, 44 a n d 64. W h e r e t h e b a n d s h a v e clean a n d a p p r o x i m a t e l y s y m m e t r i c a l profiles, t h e r.m.s, v a r i a t i o n in t h e c a l c u l a t e d p e a k v a l u e s is u s u a l l y less t h a n 0.35 e m -1. I n m o s t i n s t a n c e s w h e r e larger r.m.s, v a r i a t i o n s occur, i n s p e c t i o n of t h e original curves i n d i c a t e s t h e p r e s e n c e of " s p i k e s " due to w a t e r v a p o r a b s o r p t i o n ( B a n d s 3, 4, 45, 46, 54). On t h e basis of this analysis, t h e b a n d s m o s t s u i t a b l e for c a l i b r a t i o n p u r p o s e s are p r i n t e d in boldface t y p e in T a b l e 1. Most of these h a v e r.m.s, v a r i a t i o n s * b e l o w 0-35 c m -1. B a n d s 3, 4, 5, 8 a n d 15 for w h i c h t h e r.m.s, v a r i a t i o n e x c e e d s 0.35 c m -1 are utilized b e c a u s e no a l t e r n a t i v e b a n d s are a v a i l a b l e to bridge t h e g a p s b e t w e e n 3800 a n d 3100 c m -1 a n d b e t w e e n 3015 a n d 2770 c m -1. F o r m o r e a c c u r a t e c a l i b r a t i o n of high r e s o l u t i o n s p e c t r o m e t e r s t h e v a p o r p h a s e b a n d s of H~O, COs, H C N a n d CH~ can be u s e d o v e r t h e r a n g e s 3800-3580 c m -1 a n d 3300-3100 c m -1, b u t t h e r a n g e 3580-3300 c m -1 is also p o o r l y s e r v e d b y v a p o r p h a s e s p e c t r a (3). More a c c e p t a b l e c a l i b r a t i o n s t a n d a r d s are b a d l y n e e d e d for this r a n g e in v i e w of its i m p o r t a n c e in studies of h y d r o g e n b o n d i n g of O - - H a n d N - - H groups. F o r t h e m o r e a c c u r a t e c a l i b r a t i o n of t h e r a n g e 3015-2770 c m -1 H C I can c o n v e n i e n t l y be e m p l o y e d [3].

Acknowledgement--We are grateful to Dr.

P. J. KRUEGER of the University of Alberta at Calgary for making available to us his numerical data reproduced in Table 1.

* ~I'he r.m.s, variation is given by [(~ -- Vobs)2/n]½where ~ = E~obs/n is the individual peak measurement and n the number of observations.