5457
J. Phys. Chem. 1995, 99, 5457-5464
Molecular Modeling of Melamine -Formaldehyde Resins. 2. Vibrational Spectra of Methylolmelamines and Bridged Methylolmelamines Robert J. Meier* DSM Research, P.O. Box 18, 6160 MD Geleen, The Netherlands
Andrew Tiller Biosym U.K., INTEC Technology Centre, Wade Road, Basingstoke, Hampshire RG24 ONE, U.K.
Sylvia A. M. Vanhommerig Laboratory of Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Received: July 13, 1994@
A recently developed force field has been employed to interpret the vibrational spectra of methylolmelamines and their bridged oligomers in melamine-formaldehyde adducts and resins. The results show that some surprising experimentally observed features, such as the apparent insensitivity of one of the ring breathing modes and the very strong sensitivity of the other ring breathing mode upon methylolation, can be fully recovered from the calculations. Most, although not all, assignments based on the present calculations are in agreement with previous assignments based on purely experimental data. It is thus shown that vibrational analysis with a good quality force field can significantly contribute to a further understanding of the spectra of complex systems such as melamine-formaldehyde resins.
1. Introduction In their 1982 book' The theory of vibrational spectroscopy and its application to polymeric materials, the authors Painter, Coleman, and Koenig wrote, "Although there is certainly no better help than extensive practical experience, it is also true that a fundamental understanding of the vibrational process in polymers is the ultimate foundation of efforts to interpret spectra. Fortunately, the theoretical aspects required for this understanding are available and with modem digital computers, one can take full advantage of these techniques. Unfortunately, except in a few laboratories, the fundamental understanding and techniques do not exist." It seems that up until the present day this situation has not really changed. In this paper we will pursue such a theoretical analysis applied to a problem of considerable practical industrial interest. Both urea and melamine are ingredients in an industrially very important class of urea-(melamine-)formaldehyde resins, with applications, for example, in fiberboard. In the present paper we will focus on melamine-formaldehyde resin. The first step in the preparation of such a resin is the reaction of the amine group of melamine with formaldehyde to form methylol groups, e.g.,
NHz
NH2
All six hydrogens in melamine can be successively replaced by methylol groups. Thus, the family of methylolmelamines comprises the nine species shown in Figure 1. In order to distinguish between the different di-, tri-, and tetramethylolmelamines, we have adopted the notation of Gordon,2 i.e. [m,n], with m denoting the number of substituents on a secondary nitrogen atom and n denoting the number of substituents on a tertiary nitrogen atom. @Abstractpublished in Advance ACS Abstracts, March 1, 1995.
4
4-
12.01
[OPl
Figure 1. Family of the methylolmelamines, comprising nine different species, labeled according to the notation of Gordon: i.e. [m,n],with m denoting the number of substituents on a secondary nitrogen atom and n denoting the number of substituents on a tertiary nitrogen atom.
The propagation step in resin formation is the creation of methylene andor ether linkages between the methylolated melamines. Denoting a melamine ring by @, @NH2
+ HOC(H2)NH-@+N(H)C(H,)NH+
+
methylene bridge
-
@-N(H)CH,OH HOC(H2)NH-@ 4-N(H)CH,OC(H,)NH-@
(2) ether bridge
Experimental characterization of the formation of methylene and ether bridges is very difficult. At the molecular level these resins have a network structure which is not yet well-known.
QQ22-3654l95l2Q99-5457~Q9.QQlQ0 1995 American Chemical Society
Meier et al.
5458 J. Phys. Chem., Vol. 99, No. 15, 1995
Infrared spectroscopy is problematic due to the large number of different structures present in the methylolmelamine family and broadness of the absorption bands. Raman spectroscopy is considered a more suitable tool, the bands being usually much narrower and thus less strongly overlapping than in IR spectra. In this respect we refer to a paper by Hill et al.,3 who studied urea-formaldehyde resins employing Raman spectroscopy. More recently Scheepers et aL4 have made substantial progress in characterizing the melamine-formaldehyde system. By using Raman spectroscopy in conjuction with 13C NMR and HPLC, they were able to show that at low pH and low formaldehyde to melamine (F/M) ratio there is a predominance of methylene bridge formation, whereas at high pH and high F/M ratio ether bridges are formed as well. This information is of great importance in characterizing the network structure of melamineformaldehyde resins. We have adopted the point of view that theoretical calculations could assist in the full interpretation of spectroscopic data. In order to be able to perform force field calculations yielding vibrational spectra of good quality, we need a force field of good quality. The latter was previously developed within the context of a new contribution to the CFF91 force field incorporated in the DISCOVER program.6 This development required the determination of a set of force constants for melamine which is consistent with the existing part of the CFF91 force field. This consistency ensures that the force field can be employed to study, for example, substituted melamines. The newly derived force field was explicitly tested on melamine in part 1 of this series: in particular the vibrational spectrum of melamine, which also contains the newly derived parameters for melamine. Apart from the paper by Scheepers et al. there are no published data on the vibrational spectra of the methylolated melamines nor of the cured (methylene/ether bridge formed) resin. In an attempt to interpret their spectra, Scheepers et al. had to rely partially on published data for ureaformaldehyde species and also on IR data. Although such reference data are evidently not the a priori choice for interpreting the melamine-formaldehyde spectra, these authors had no choice in view of the absence of any other more relevant reported data. Moreover, they noted in their conclusions that “it is also clear that there are still regions in the Raman spectra of MF resins where the interpretation remains tentative.” The present paper is the first to present theoretical frequencies on the methylolmelamine and bridged methylolmelamine species. The focus will be primarily on the spectral ranges studied by Scheepers et al., i.e. the ranges 600-1500 cm-’ and 28003100 cm-’. The region below 600 cm-’ was not considered because in that range lattice modes are also present; these cannot be described using single-molecule calculations. Trends observed when varying the degree of methylolation will be discussed and compared to the experimental observation^.^ Furthermore, because the force field does not allow for calculation of Raman intensities, we have particularly focused on changes that occur upon methylolation. When the character of the mode does not undergo a major change, the Raman activity can reasonably be assumed to persist in the methylolated species. For comparison, Figure 2 shows the experimental Raman spectra of melamine, a mixture of methylolmelamines prepared from a F/M ratio of 2: 1, and a mixture of methylolmelamines prepared from a F/M of 6:l. The species resulting from the 2:l F/M will be primarily mono-, di-, and trimethylolated melamine: whereas the 6:l ratio results in a mixture rich in highly methylolated melamines, albeit not exclusively the hexamethylolmelamine (for further comments see ref 4). The spectra have been displaced here on a somewhat more detailed
1100
1000
800
900
700
600
Raman shift l c m . ’ l
6
1ioo
1600
1500
1400
iioo
1200
1100
1000
Raman shift lcm”)
dbL
-L 3M0
3+00
3300
3200
3100
Ranan a h i f t
3000
2900
2800
(0.’)
Figure 2. Experimental Raman spectra for melamine (bottom), methylolmelamines prepared from a melamine-to-formaldehyderatio of 2: 1 (middle), and methylolmelamines prepared from a F/M ratio of 6:l (top). The spectrum has been subdivided into three ranges (a) 5001200 cm-’, (b) 900-1800 cm-’, and (c) 2800-3500 cm-’.
scale than was done with similar spectra in ref 4,and the three spectra shown illustrate typical trends with increasing methylolation. 2. Computational Details
All force field calculations reported below were camed out with the full force field reported in ref 5 using DISCOVER
J. Phys. Chem., Vol. 99, No. 15, I995 5459
Molecular Modeling of Melamine-Formaldehyde Resins
TABLE 1: Character Table of the Point Group D ~ I , D3h E 2c3 3c2 Uh 2s3 3Uv A ' 1 1 A'2 1 E' 2 A"l 1 A"2 1 E" 2
1 1 -1 1 1 -1
1 -1 0 1 -1 0
1 1 2 -1 -1 -2
1
1 -1 -1 -1 1
1 -1 0 -1 1 0
xZ+y2,22
R, (xy)
(9- y 2 , x y )
(b)676 Cm" (c) 670 cm"
z (R,,R,)
A-
/-
(XZ,YZ)
version 2.9 running on a Silicon Graphics 4D/35 workstation. The structures were displayed using INSIGHTII version 2.2.(16 The eigenvectors were displayed using the available software tools in this package for further characterization of the vibrational modes. Force field calculations in their present form are not capable of handling Raman intensities. However, knowing the eigenvectors (displacement vectors of the atoms during normal mode vibration) as obtained from the force field frequency calculation, it is possible to calculate the IR and Raman activities from the transformation properties of the eigenvectors under the operations of the symmetry group.' For melamine, assuming it to be planar for the moment, the symmetry group is D3h. Although we now know that the minimum energy structure is nonplanar,* averaging over the time scale of vibrations, the structure is effectively planar. Table 1 contains the relevant character table. From Table 1 it is seen that, for instance, all degenerate vibrations (E-type vibrations) are Raman active. Because the Raman intensities depend on the derivatives of the polarizability with respect to displacement, they are, in principle, extremely sensitive to changes in structure and character of the eigenvector. Therefore, changes in the character of the mode (i.e. eigenvector or atomic displacements during vibration) will have a large effect on the Raman intensity, in particular when a rather symmetrical displacement is distorted. This argument can be used to explain the disappearance of Raman intensity in certain bands upon methylolation, as we will demonstrate in section 3. Ab initio calculations at the 6-31G*//6-31G* level (Le. 6-3 lG* calculations on 6-3 lG* optimized geometry) were carried out on melamine and monomethylolmelamine. For this purpose the Gaussian 909 suite of programs was used as installed on an Alliant FX-2816 computer at Eindhoven University of Technology. Frequencies as well as IR and Raman intensities were calculated after full geometry optimization.
3. Results and Discussion 3.a. Methylolmelamines. 675 cm-' Band. One of the most intense Raman bands in melamine is the 675 cm-' band, which corresponds to the calculated frequency at 645 cm-' and is attributed to the in-plane ring deformation of the triazine ring, viz., Figure 3a. Scheepers et al.4 observed that this band was practically absent in all methylolmelamines. The vanishing band intensity upon methylolation seems surprising and strongly contrasts with the persisting intensity of the 975 cm-' ring mode upon methylolation. We have analyzed all the normal modes with a frequency in the range of that of the unsubstituted melamine. For the methylolmelamines, all modes in this range have eigenvectors entirely dissimilar to that of the 675 cm-l mode in melamine. The strong change in character of the vibration and, in particular, the accompanying loss of symmetry upon methylolation account for a change in Raman intensity. However, this does not necessarily explain the vanishing Raman intensity upon methylolation. An ab initio calculation on monomethylolmelamine, including a frequency analysis and calculation of the IR and Raman intensities, revealed that for the experimental 984 cm-' ring breathing band the ab initio calculated frequency only varied by 2 cm-' when going from melamine to monomethylolmelamine with an accompanying
4 4 (h) 651 Un"
(j) 612 cm"
2
(k) 670 cm"
-4-$
1 (I) 668 c"'
3
\
(m)623 cm" (n) 606 cm"
Figure 3. (a) Eigenvector of the 645 cm-I (experimental 675 cm-I) ring breathing mode in melamine; (b-f) eigenvectors of vibrational modes of monomethylolmelamine covering the range 578-676 cm-'; (g-j) eigenvectors of those vibrational modes of trimethylolmelamine[3,0]closest to 645 cm-'; (k-n) eigenvectors of those vibrational modes of hexamethylolmelamine closest to 645 cm-I; all eigenvectors displayed in Figure 4(b-n) are of very dissimilar character to the 645 cm-l mode of melamine.
5460 J. Phys. Chem., Vol. 99, No. 15, 1995
TRIMETHYLO1.MELAMINE
rY
HEXAMETHYLOLMEUMINE
2-
Figure 4. Eigenvectors and frequencies for the ring breathing mode, observed experimentally at around 984 cm-I, as calculated for melamine (a), monomethylolmelamine (b), trimethylolmelamine[3,0](c), trimethylolmelamine[ 1.21 (d), and hexamethylolmelamine (e).
increase in Raman intensity of about 10%. In contrast, the total intensity is constant for the calculated band corresponding to the experimental 675 cm-' band for monomethylolmelamine as compared to the band in melamine, but this intensity is distributed over two (calculated) bands, one 30 cm-' above the 675 cm-' melamine band and one 30 cm-' below the corresponding melamine band. These data therefore also suggest that the intensity at 675 cm-' vanishes upon methylolation and suggest that the intensity may be further smeared out with increasing methylolation. 811 cm-' Band. In melamine a moderately strong IR absorption band has been observed at 811 cm-', with a calculated frequency at 829 cm-'. This mode involves in-phase out-of-plane motion of the carbon atoms perpendicular to the triazine ring plane and in-phase out-of-plane motion of the triazine nitrogen atoms 180" out-of-phase with the carbon atom motion. The calculations on the methylolated melamines unmistakably show the persistence of this band, with the same eigenvector and thus with sustained IR activity. The calculated frequencies are 829 (melamine), 833 (monomethylolmelamine), 840 (trimethylolmelamine[3,0]),and 843 cm-' (hexamethylolmelamine), respectively. We were therefore able to predict the persistence of this IR band upon methylolation, which has since been confirmed by experimental IR analysis. lo 984 cm-I Band. The experimental frequency at 984 cm-' in melamine is due to the ring breathing mode displayed in Figure 4a and compares with the calculated frequency of 966 cm-'. The experimental frequency of this mode upon methylolation is constant. The character of this mode (involving a vibration in the triazine ring which has to "carry" the substituent methylols with it) initially suggests that the higher mass on the ring and the lowering of symmetry would seriously affect both frequency and intensity of this band. The frequencies for this ring mode were calculated, however, as 966, 969, 972, and 981 cm-' for the monomethylolmelamine, the two types of trimethylolmelamine, and hexamethylolmelamine, respectively, the corresponding structures and eigenvectors being displayed in Figure 4. The force field calculations thus substantiate the assignment of these bands to the ring breathing mode, and the result that the character of the mode is also unchanged accounts for the constant Raman intensity upon methylolation. 999-1350 cm-' Region. For melamine the only experimentally observed band in the range 999-1030 cm-l is an infrared absorption band with moderate intensity near 1028 cm-', which was previously identified with the force field calculated degenerate doublet at 1025 ~ m - ' . The ~ calculated 1098 cm-'
Meier et al. band has been tentatively assigned to the experimental shoulder observed near 1075 cm-', but this must be considered disputable. Awaiting a more definite assignment, the experimentally observed bands in methylolmelamines have been attributed to methylol group^.^ In the experimental Raman spectra, bands were observed at 999 cm-' and at 1015-1020 cm-', with the latter shifting to 1030 cm-' at high F/M. For melamine itself all six calculated frequencies in the range 1025- 1142 cm-I (the latter value involves a doublet) are related to normal modes characterized by combined motion within the triazine ring and the NH. The persistence of either Raman or IR activity when the melamine is substituted by methylol groups depends on the symmetry of the vibrational motion in the methylolated melamines. Formally, symmetry arguments employed for melamine do not apply to monomethylolmelamine because a single substitution of a methylol group implies the loss of all overall symmetry in the system. However, local or site symmetry might be conserved. In fact the change in IR or Raman intensity depends on the change in dipole moment or polarizability, respectively, and consequently the conservation of local symmetry in cases where the molecular motion involved in the vibration is also local at the same place in the molecule will still exhibit the same IR or Raman intensity as in the unsubstituted melamine. The force field calculated frequencies in the aforementioned range for monomethylolmelamine at 1037, 1063,1122, and 1147 cm-' all involve the methylol group to a certain, although sometimes limited, extent, whereas modes 1173, 1203, and 1303 cm-' are all pure methylol group modes. For trimethylolmelamine[3,0] the methylol group related modes occur at 985 cm-' and in the (calculated) range 1173-1392 cm-l. At 1114 cm-', however, another mode is found which is doubly degenerate and transforms according to an E' representation, and therefore is potentially Raman active. The mode is a combination of ring motion with methylol group CH stretch. For hexamethylolmelamine the calculated frequencies related to methylol group motion all lie in the range 1096-1267 cm-'. An additional band at 1044 cm-' is calculated which involves both ring and methylol group motion with the eigenvectors strongly inferring A'1-type motion, Le. totally symmetric. The fact that the experimental spectra show that the new bands arising upon methylolation are shifted by less than 100 cm-' upward from the symmetric "984 cm-'" breathing motion band for all these species leads to the conclusion that these modes cannot be unambiguously assigned to pure methylol groups. The modes are rather a combination of motion within a methylol group and the triazine ring. This does not preclude that the integrated band intensity be used, albeit rather pragmatically, for quantifying the methylol group content of methylol melamine mixtures, as proposed in ref 4. For low F/M ratio the observed band at 999 cm-' should probably be identified with the calculated 985 cm-' band for trimethylolmelamine[3,0].For hexamethylolmelamine this extra band is not found in the calculations, in agreement with experimental observations (Figure 2a). I443 cm-I Band. Whereas experimentally melamine shows a band at 1443 cm-', the methylolated mixtures prepared from F/M = 2 and F/M = 6 exhibit a band at 1460 cm-I and another band at about 1390 cm-'. For F/M = 2 a very low intensity Raman band is observed at about 1350 cm-'. The calculated 1461 cm-I band for melamine displays the character of a ring breathing mode (Figure 5). The 1443 cm-' band for (unsubstituted) melamine was identified with the force field calculated 1485 (degenerate doublet) and 1461 cm-' frequencies and predominantly involves C, motion. These bands have some
Molecular Modeling of Melamine-Formaldehyde Resins
7
Figure 5. Eigenvector of the calculated 1460 cm-' mode in melamine which corresponds to one of the modes in the experimentally observed envelope centered near 1443 cm-'.
correspondence in monomethylolmelamine, where modes at 1487 and 1478 cm-' show significant resemblance to the 1485 cm-' doublet in melamine. In trimethylolmelamine, however, the situation is completely different. Whereas in melamine and monomethylolmelamine the eigenvectors mainly show Cmazine motion, for trimethylolmelamine[3,0] this changes to CH2 motion in the methylol groups. The latter are likely to be Raman active (based on site symmetry rather than overall symmetry), and the calculated range now extends from 1474 down to 1440 cm-'. There is a ring mode at 1437 cm-' which is possibly related to the 1460 cm-' mode in melamine, followed by CH modes extending down to 1392 cm-'. Although we have no Raman-calculated intensities available for the trimethylolmelamine[3,0] species, the calculated data and the high probability that CH vibrations are Raman active agree with the broadening of the range of frequencies in this region from 1443 cm-' for melamine to 1390-1470 cm-' for trimethylolmelamine[3,0].For hexamethylolmelamine we found frequencies at 1484 and 1481 cm-' that are dominated by Cuiazlne-Naminestretch and CH frequencies extending further down to 1412 cm-'. The spectrum due to CH vibrations in hexamethylolmelamine appears to extend less far to low Raman shifts (1412 cm-') than in trimethylolmelamine (1390 cm-'). This is in agreement with the experimental observation (Figure 2b) of further extension of Raman intensity for a F/M = 2 than for a F/M = 6. Further statements can, we believe, only be made when Raman intensities are calculated, which at present is too computationallydemanding for all of the methylolated melamines. I550 cm-I Bund. Melamine exhibits an envelope centered at 1550 cm-' in the experimental Raman spectrum, which effectively covers the range 1500-1590 cm-'. The corresponding calculated bands were found at 1599, 1592, and 1591 cm-'. Because the band is both IR and Raman active, the latter two calculated frequencies form a degenerate doublet of E'-type. For the methylolmelamines the character of these modes changes dramatically upon methylolation. While for melamine itself we found extremely well-defined NH-bending combined with CNstretching motion for all three modes in this range, in monomethylolmelamine only two out of the three frequencies retain this character. For tri- and hexamethylolmelamine, however, the eigenvector analysis shows the motion to be located increasingly in the methylol groups. The decrease of symmetry of the overall species upon methylolation accounts well for the loss in Raman intensity of these modes which is observed experimentally. 1650 cm-' Bund. The calculated frequencies (doublet) for melamine at 1652 cm-' were related to the experimentally observed strong IR and very weak Raman band at about 1650 ~ m - l . ~The eigenvectors of the corresponding modes in melamine, monomethylolmelamine, trimethylolmelamine[3,0], and hexamethylolmelamine show that, with increasing degree of methylolation, small differences occur with respect to the
J. Phys. Chem., Vol. 99, No. 15, 1995 5461 degeneracy of the mode and, more importantly, the character of the eigenvectors. Since the symmetry of the eigenvectors diminishes when going from melamine to hexamethylolmelamine, the Raman intensity (if the original band was Raman active) is expected to decrease, as is indeed observed experimentally (Figure 2b). 2800-3500 cm-l Region. Melamine shows a broad band centred around 3120 cm-', with a weaker additional feature near 3320 cm-', and two sharp bands around 3420 and 3470 cm-'. It is known that the latter two bands are due to free h-2 groups, whereas the former broad features originate from interacting NH2 groups. In the spectrum taken from the methylolmelamine mixture prepared from a F/M ratio of 2, a broad feature extending from 3050-3350 cm-' is still visible. This feature is sharper in spectra recorded on other samples of methylolmelamines prepared at high F/M ratio.1° The intensity of the broad feature in the range 3050-3350 cm-l is therefore, we believe, not background but a true part of the spectrum and is assigned to secondary amines. This is further evidenced by the fact that for the methylolmelamine mixture prepared from a F/M ratio of 6 the broad feature has strongly decreased in intensity compared to the mixture prepared from a F/M of 2. Moreover, for the cured resins prepared from FA4 ratios 2 and 6, respectively, we observe the same background as for the uncured systems, Le. a clearly visible background in the 30503350 cm-' region for F/M = 2 and a strongly reduced background for F/M = 6. This at least suggests the feature is not due to methylol groups, for in that case the band would be expected to be subject to change upon bridge formation. For the methylolmelamines three bands are found experimentally in the range 2800-3100 cm-', i.e. at 3000 cm-', at 2966-2977 cm-', and in the range 2896-2909 cm-'. Mixtures of methylolmelamines prepared from higher F/M ratios show a distinct band at 3015 cm-'. The spectra shown in ref 4 leave open the possibility that the 3015 cm-' band could be interpreted as a further shift in the 3000 cm-' band found at lower F/M ratios. For melamine the force field calculations revealed no bands in this range, which is in accordance with the experimental spectrum (Figure 2c). The monomethylolmelamine and both types of trimethylolmelamine show two distinct calculated bands at 2888 and 2970 cm-'. Pentamethylolmelamine and hexamethylolmelamine show four closely spaced bands at 28642868, 2885-2894, 2917-2922, and 2960-2968 cm-'. All these calculated frequencies were characterized as either CH or CH2 vibrations, in agreement with the interpretation given by Scheepers et a1.4 The long tail to lower Raman shifts in the experimental spectrum for the 2896-2909 cm-' band supports the view that the two lower calculated regions (2864-2868 and 2885-2894 cm-') form this envelope. So if the 3015 cm-' band is interpreted as the further shifted 2966-2977 cm-', the agreement between theory and experiment is good. We initially postulated that the 3015 cm-I band originates from the formation of bridged methylolmelamine species, viz., reaction 2,'' but this option must be ruled out because in this range no calculated frequencies above the 2970 cm-' band were found at all for the bridged species. 3.b. Bridged (Condensated)Methylohelamines. The frst step in the creation of a melamine-formaldehyde resin network is the formation of ether and/or methylene bridges; see eq 2. We have studied hexamethoxymethylmelamine, a species consisting of two ether-bridged melamine molecules, and methylenebis(pentamethylolme1amine) as model systems for studying the vibrations in these bridged species. The three structures are depicted in Figure 6. The ether and the methylene bridges will be discussed separately. We will focus on bands
5462 J. Phys. Chem., Vol. 99, No. 15, 1995
Meier et al.
J P " H
H
Figure 6. The model systems employed for studying the vibrational spectra of bridged methylolmelamines: (a) hexamethoxymethylmelamine,(b) a species consisting of two ether-bridged melamine molecules, and (c) methylenebis(pentamethylolme1amine).
that are related to the experimental results that have been reported recently, rather than trying to cover the full range of frequencies. Scheepers et aL4 concluded that, on the basis of the study of model compounds, differentiation between OCH20 (1490 cm-'), NCH20 (1450-1460 cm-'), and NCH2N (1435 cm-') is possible. The OCH20 group appears exclusively in the paraformaldehyde and formalin model compounds they have employed and will not be discussed in the present paper. The NCH2N appears exclusively in the methylene-bridged species, whereas the NCH20 group is present in both the methylolmelamines and in the ether-bridged species. 3.b.l. The Ether Bridge. The typical ring breathing mode which is found in melamine at 966 cm-' (calculated frequency) and at 981 cm-' in hexamethylolmelamine was found at 979 cm-' for the ether-bridged species hexamethoxymethylolmelamine. The eigenvectors are identical to those of the corresponding modes in melamine and all of the methylolmelamines (see discussion above), and consequently the character of this mode does not change upon ether bridge formation. Scheepers et al., whilst also refemng to assignments proposed by Hill et aL3 for the urea-formaldehyde system, assigned a band at 910 cm-' to a C-0-C symmetry stretch. In aliphatic ethers two bands in the 1000 cm-' range are conventionally assigned to the symmetric and asymmetric COC stretch vibrations,12 and a COC symmetric deformation is assigned around 430 cm-'. It should be mentioned that these early assignments, in particular the character of the vibrations, were, at least partially, based on a force field analysis. The general quality of force fields was not very good at that time. This has improved considerably since, and our recently developed force field5 is likely to be the best available at present for urea- and melamine-containing molecules. In order to further investigate the interpretation of the 910 cm-' band, we examined modes in the calculated frequency range 979-867 cm-' for hexamethoxymethylolmelamine. Bands were found at 979 cm-' (this frequency coincides, according to the calculation, with the ring breathing mode) and 972 cm-', which resembles COCH motion; see Figure 7a,b. Bands calculated at 924, 919, 906, and 876 cm-' are all pure CH2 vibrations, more precisely the CH2 bonded to the amino nitrogens (Figure 7c-f). At still lower Raman shifts, the next six calculated bands involve ring motion accompanied by some side group motion. This interpretation might be disputed because the isolated CH2 motions would also be expected in the methylolmelamines, for which no band is observed. Now for the methylolmelamines prepared from a mixture of F/M = 6 (pH = 9), which should mainly lead to
Figure 7. Calculated eigenvectors and frequencies in the range 870980 cm-' for the ether-bridge-containing species hexamethoxymethylmelamine. For further comments see text in section 3.b.l.
hexamethylolmelamine if no formaldehyde is lost during the synthesis process, a band is observed in this range.4 However, it is believed" that some bridge formation has already taken place in the highly methylolated melamines, and so we might adopt the former assignment4 for the cured systems, i.e. a C-0-C stretch vibration. For the methylolmelamine mixture prepared with F/M = 1.7 a band around 910 cm-' is hard to discern; it was shown by means of NMR (ref 4, Table 2, material MF7e), however, that this material preferentially forms methylene bridges when prepared at pH 7.6 (at high pH more ether bridges are found using NMR,9 but no experimental spectra were reported in ref 4). This gives further evidence for the assignment
Molecular Modeling of Melamine-Formaldehyde Resins of the observed band at 910 cm-' to ether linkages in the methoxymethylmelamines and the highly substituted methylolmelamines involving partial condensation. Although our theoretical results do not support this view, the above discussion seems to favor the current interpretation that the 910 cm-' should be characterized as a C-0-C stretch vibration. However, analysis of the vibrational modes for hexamethylolmelamine, trimethylolmelamine[3,0], and monomethylolmelamine yields surprising results. For hexamethylolmelamine calculated bands were found at 955,948,924,910, and 901 cm-', all of which involve CH2 motion. The character of the eigenvectors is basically identical to those in the hexamethoxymethylmelamines. For trimethylolmelamine[3,0] the calculated frequencies in the same range were found at 958, 957, 922, and 873 (doublet) cm-'. The first three modes have eigenvectors displaying predominantly NCH motion, i.e. the amino nitrogen, the bonded C of the methylene group, and one of the methylene protons. The doublet at 873 cm-' involves both ring and side group motion. For monomethylolmelamine calculated frequencies are 960 and 892 cm-', with'eigenvectors displaying NCH2 motion, as for the trimethylolmelamine[3,0]. We conclude that the character of the vibrations in this frequency range clearly changes when going from highly substituted (hexamethylolmelamine and hexamethoxymethylmelamine)to the less substituted methylolmelamines. As a consequence, we are tempted to believe that our theoretical interpretation that the 910 cm-' band in hexamethoxymethylmelamine is due to localized CH2 motion rather than a C-0-C stretch is not at variance with experimental data obtained on the series of methylolated melamines. Moreover, further evidence for this interpretation is obtained from a recent study on tea-butyl ethyl ether which involved gas-phase electron diffraction, ab initio calculations, and vibrational spectroscopic data.l 3 In that study, bands in the region around 910 cm-' were found to involve a combination of C-C asymmetric stretch and CH3 rocking motion, whereas a band around 960 cm-' was characterized by 0-C stretch and CH3 rocking motion. Motions involving the entire C-0-C unit were only found in the bending region below 530 cm-'. In order to provide additional support for our proposed interpretation, we studied the spectrum of a molecule formed by two melamines bridged by one ether bridge (Figure 6b). The four vibrational frequencies just below the ring breathing mode at 966 cm-' are found in the range 831-910 cm-'. All four bands and the ring breathing modes for the species are displayed in Figure 8. It appears that only the two central frequencies at 910 and 882 cm-' are related to the bridge. However, the two bands are at different frequencies (determined by differences in local environment) and involve almost pure CH2 motion. This further confirms our assignment of the 910 cm-' band to CH2 group motions. Now that we have identified the 910 cm-' band with CH2 group vibration, one might want to investigate the frequency shifts induced by different neighboring atoms. For example, the potential field of a neighboring atom could induce a frequency shift making the band position characteristic for the CH2 group in the given local environment. In the ether bridge we are studying, the CH2 can be influenced by both the neighboring N and 0 atoms in the NCH20 unit. One might then ask what the difference is between this and the NCH20 frequency assigned by others to the experimentally observed 1450-1460 cm-' band. Further analysis seems required to explain in full which groups are responsible for the various characteristic frequencies. Nevertheless, the conclusion that the 910 cm-' band cannot be assigned to a C-0-C stretch stands. This result implies that Raman spectroscopy on its own cannot
J. Phys. Chem., Vol. 99, No. 15, 1995 5463 me
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\
it
%Figure 8. Calculated eigenvectors of a species consisting of two melamines bridged by an ether linkage. The calculated vibrational frequencies are 966 cm-' (FRQ051 and FRQ 052), 910 cm-l (FRQ 053), 881 cm-' (FRQ054), 832 cm-I (FRQ055). and 831 cm-I (FRQ 056). The eigenvectors of the modes 051, 054, and 055 have been displayed above the horizontal line, whereas the eigenvectors of the modes 052, 053, and 056 are shown below the horizontal line. FRQ 051 and FRQ 052 are the ring breathing modes, and FRQ 053 and FRQ 054 are CH:! vibrations formerly assigned to a C-0-C stretching vibration.
provide unambiguous data relating to ether bridge formation. Scheepers et alS4have shown that unambiguous assignment can be accomplished by combining Raman spectroscopy with other techniques, such as HPLC or 13C liquid NMR, although these techniques are not applied to solid state materials, for which Raman spectroscopy remains the only tool. For the methylolmelamines the bands in the range 10001030 cm-' (experimental range) were assigned to methylol groups. Our calculations reveal that modes which are due to methylol groups do appear in this range, although the calculated frequencies are somewhat higher. These bands were assigned to methylol groups because they vanish upon bridge f ~ r m a t i o n . ~ However, the figures in ref 4 show that only for the methylolmelamines with low F/M (F/M = 1.7) does the intensity vanish entirely upon bridge formation; for F/M = 6 the intensity is reduced by 70%. For a full interpretation of the spectra, including the dependence on F/M ratio, we need access to calculated Raman intensities. Since these cannot be obtained other than from prohibitively expensive ab initio calculations, we make no further comments here. The band in the range 1450- 1460 cm-' has been attributed to the NCH20 entity.4 We have already discussed the calculated frequency range 1342-1569 cm-', and we no conclude that the character of the 1450-1460 cm-' band is primarily related to CH2 group motion, in line with the former discussion of the 1443 cm-' region for the methylolmelamines. Our calculations reveal that the character of the bands in the 1550 cm-' region does not change significantly between the hexamethylolated melamine and the ether-bridged species linking two of these hexamethylolated melamines. This observation is consistent with the experimentally observed persistence of the band intensity upon curing (=bridge formation) as reported by Scheepers et al. (Figure 4, ref 4). 3.b.2. The Methylene Bridge. The experimentally observed band at 1435 cm-' has been assigned4 to CH2 vibration in NCH*N, a conclusion which was partially based on the study of spectra of reference compounds such as methylene-diurea, Within the range 1337-1537 cm-', our calculations on methylenebis(pentamethylolme1amine) reveal a vibrational mode at 1513 cm-I, characterized as a CH2 vibration in the methylene bridge. Two vibrations are found in this frequency range
Meier et al.
5464 J. Phys. Chem., Vol. 99, No. 15, 1995
involving Cring-Namino motion, where the CN is part of the methylene linkage between the triazine rings. The calculated frequencies are 1468 and 1480 cm-'. The difference between the calculated (1513 cm-') and experimental (1435 cm-') frequencies is 78 cm-', which is still within the limits of accuracy we reported for melamine.5 The result that the experimentally observed frequencies should be interpreted as motion within a CH2 group does not imply that the frequencies cannot be characteristic of a larger molecular group, in the present case NCH2N. The interatomic potential between the CH2 group and the neighboring nitrogen atoms must be considered capable of causing a shift in frequency, thus making the specific frequency characteristic for the larger NCH2N group. This would imply an indirect assignment to such larger moieties. The rationale here is that the neighboring atom can contribute to the potential experienced by the vibrating CH2 group, thus influencing its acutal frequency and making it characteristic for the larger unit. Further calculations are required to confirm this suggestion. According to Jada14 the 2966-2977 cm-' band is due to CH stretching modes in alcohols, ether, and NCH2N groups. Our calculations reveal that in the range 2864-2968 cm-I (the first band below this is around 1600 cm-', and above this range only the methylol OH vibrations at around 3500 cm-I are found) those eigenvectors showing CH stretching in the methylene bridge are as localized as all the other CH vibrations. The frequencies are scattered across the whole range. The Raman activity of all these modes is expected to be similar to each other. More importantly, for the purpose of characterizing the melamine-formaldehyde resins, this range seems incapable of revealing any relevant information with regard to the presence of methylene bridges.
supported by various calculations. The 910 cm-I band is therefore not characteristic of the C-0-C group, an interpretation which is not at variance with the experimental data on the methylolmelamines. The character of the corresponding band in the methylolmelamines varies appreciably with the degree of methylolation. As a consequence, the 910 cm-' band may seem to be characteristic of the C-0-C group because there is no other CH2 group in the system that could cause this band to appear in the Raman spectrum. When the constituents of the sample are well-known, the 910 cm-' band may therefore be used to characterize the C-0-C linkages. Knowledge of the constituents could be accomplished by employing additional techniques. Scheepers et al. used NMR and HPLC4 However, Raman spectroscopy alone cannot characterize the ether bridges. Any change in the preparation of the MF resin might yield different species in the reaction product and might therefore require additional characterization by other techniques such as NMR and HPLC. Nevertheless, it is important to emphasize that, in specific cases, the 910 cm-' band can be used to characterize the C-0-C linkages, a conclusion which has considerable pragmatic value.
Final Discussion and Summary
(1) Painter, P. C.; Coleman, M. M.; Koenig, J. L. The Theory of Vibrational Spectroscopy and Its Application to Polymeric Materials; John Wiley & Sons: New York, 1982. (2) Gordon, M.; Halliwell, A,; Wilson, T. J . Appl. Polym. Sci. 1966, 10, 1153. (3) Hill, C. G., Jr.; Hedren, A. M.; Myers, G. E.; Koutsky, J. A. J . Appl. Polym. Sci. 1984, 29, 2749. (4) Scheepers, M. L.; Gelan, J. M.; Carleer, R. A,; Adriaensens, P. J.; Vanderzande, D. J.; Kip, B. J.; Brandts, P. M. Vibrat. Spectrosc. 1993, 6, 55. (5) Maple, J. R.; Hwang, M.-J.; Hagler, A. T.; Meier, R. J. Part 1 of this series on the molecular modeling of urea- and melamine-formaldehyde resins, previous paper in this issue. (6) Models developed using software programs from BIOSYM Technologies Inc. of San Diego, computed with Discover and displayed using InsightII. (7) Cotton, F. A. Chemical Applications of Group Theory; Interscience Publishers: New York, 1963; Chapter 9. (8) Meier, R. J.; Coussens, B. J . Mol. Srruct. (THEOCHEM) 1990, 209, 303. (9) Gaussian 90, Revision I; Frisch, M. J., Head-Gordon, M., Trucks, G. W., Foresman, J. B., Schlegel, H. B., Raghavachari, K., Robb, M., Binkley, J. S., Gonzalez, C., Defrees, D. J., Fox, D. J., Whiteside, R. A,, Seeger, R., Melius, C. F., Baker, J., Martin, R. L., Kahn, L. R., Stewart, J. J. P., Topiol, S., Pople, J. A. Eds.; Gaussian, Inc.: Pittsburgh, PA, 1990. (10) Private communication from M. Scheepers. (1 1) Private communication from P. Brandts. At the higher formaldehyde to melamine ratios it is known from other experiments that bridges are being formed, albeit to a small extent. (12) Dollish, F. R.; Fateley, W. G.; Bentley, F. F. Characteristic Raman Frequencies of Organic Compounds; John Wiley & Sons: New York, 1973; Paragraph 3.2. (13) Egawa, T.; Moriyama, H.; Takeuchi, H.; Konaka, S.; Siam, K.; Schafer, L. J . Mol. Struct. 1993, 298, 37. (14) Jada, S. S. J . Appl. Polym. Sci. 1988, 35, 1573.
On the basis of the force field developed in Part 1 of this series, the vibrational spectra of methylolated melamines were calculated. The experimentally observed independence of the Raman frequency of the 984 cm-I band4 on the degree of methylol substitution was confirmed by the calculated vibrational spectra: analysis of the atomic displacements during vibration (eigenvector analysis) showed that the character of this mode (ring breathing) does not change upon methylolation. On the other hand the second ring mode in melamine which is observed experimentally near 675 cm-' shows a dramatic drop in Raman intensity upon methylolation. Our theoretical analysis indicated that, even for monomethylolmelamine, the character of the mode is entirely dissimilar to that of the 675 cm-' mode in melamine, a feature which accounts for the dramatic decrease in Raman intensity. Our calculations also reveal that the bands in the range 9991030 cm-' have a significant methylol component, and our analysis therefore basically agrees with previous assignments. For the bands in the range 140-1650 cm-' several vibrational modes were found to change in character upon methylolation (see previous discussion). The 2800-3500 cm-' range could, in general, be well interpreted in accordance with previous assignments. For the ether-bridged species it was shown that, despite several former interpretations, the 910 cm-' band cannot be assigned to a stretch motion of the C-0-C group. This Raman band has now been assigned to CH2 group motion, which is
Acknowledgment. Dr. Martine Scheepers of the LUC Diepenbeek (B) is gratefully acknowledged for providing us with the spectra displayed in Figure 2, and both she and Dr. Bert Kip are acknowledged for critically reading the manuscript. We also thank Alan Gorman of Biosym Technologies for providing the program for eigenvector analysis and display. The authors wish to thank Dr. Paul Brandts of DSM Research for valuable and stimulating discussions on MF chemistry and for suggesting this as a suitable area for research. References and Notes
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