Collecting and Manipulating Digital Data in an Experiment on Electronic Spectroscopy In this experiment the transmission spectra of eyanine dyes are recorded on a Beekrnan DB spectraphotometer linked by a simple two wire connection t o a Nicolet 535 signal averager. The digital data are subsequently punched out on paper tape and then transferred to the University computer where s program called BFINPUT is used to convert the transmittances t o abosrbances. The digitizer should he checked t o make sure i t yields a zero voltage signal for a transmittance of zero. In addition, the digitizer signal corresponding to 1W% T should be recorded. A digitizer dwell time of 1.2 sec/channel was used since the scan speed of the DB is 1.2 sednm. If a signal averager is not available, an inexpensive digital data acquisition system can he home-built for about $650.' Before using BFINPUT the student should examine the digital data and estimate the hand center, half width a t lie, and amplitude values for the ohsewed hands (a maximum of three component bands) since BFINPUT will call for these quantities. When the information required by BFINPUT has all been recorded, then the program BANDF is executed to carry out the Gaussian curve fitting r ~ u t i n eThe . ~ best fit for the total band envelope of l,l'-diethyl-2,T-cyanineiodide is shown in Figure 1.The three calculated component Gaussians are s h o w in Figure 2 along with the experimental spectrum. The experimentalspectrum for an 8.6 X 10-6M solution in methanol is in excellent agreement with published results. The best fit curve of Figure 1was obtained after ten cycles of iteration, and the standard deviation is 0.0133.
:&
SBm,
smm
e
I
:! s.
5-s
z
z m : E?
.a.
3..
:?.
m 3mB
3am
I
zsm . I a,,
C.
9..
...
3,.
... ... ... ..
sw x,m WRVELFNETH < R > Y.
B ..
Figure 1. An experimental absorption spectrum of 1.1'diethyl-2.2'-cyanine iodide and the wrnputer-calculated fit (smooth curve) for the band envelop.
I
i
"' ,mm
.mS
"Zrn
q-n
3m
3-a
sm* szm
m m WRVELENETH
5-B
sm
SBS
Emm
Figure 2. An experimental absorption spectrum of 1.1'diethyl-2.2'-cyanine iodide. The smwth curves are computercalculated wmponem Gaussians m r r q c m i i n g to l k three vibranic transitions.
The program BANDF contains within i t a suhroutine for the numerical calculation of the integral J4w)dw where e is the molar absorptivity and w is the wavenumber in cm-'.This was used to obtain the oscillator strengths for the transitions using standard formula^,^ and these experimental quantities may be compared with those calculnted from the theoretical equations of K ~ h nComparisons .~ of calculated and theoretical values of ,A, may also be carried out. Copies of programs may he obtained from the authors. This work was supported by a grant from the National Science Foundation.
L. F., Bowers, L. D., and Cam,P. W., J. CHEM. EDUC., 43,785 (1976). (a) BANDF is a third generation modification of a program originally described by Fraser, R. D. B., and Suzuki, E., A n d Chem., 38,1170 (1966). We worked from arevised editionsupplied by D. A. Lightner and L. Cahan. (b) For analyses of the electronic spectra of aromatics see T. Kurucsev, J. CHEM. EDUC., 55,128 (1978). Pitzer, K. S., "Quantum Chemistry", Prentiee-Hall, New York, 1963, pp. 260-70. Kuhn, H., J. Chem. Phys., 17,1198 (1949). 1 whiting,
University of Nevada, Reno Reno,Nevada 89557
R. D. Burkhart P. N. Howells
Volume 56, Number 4, April 1979 / 249
